Let's go through a few things here that might save you a TON of money....

First, if the light is flashing do not drive the car.  A lit check-engine light can be serious but also not-so-much.  Anything emissions related only will light it, but there is no impact on drivability or risk to the engine in many of these cases.  A flashing check-engine light means the computer has detected a serious misfire and continued operation is likely to damage or destroy the engine.  I am assuming it is lit, not flashing, for the rest of this although in some cases a flashing light once in a while (the rest of the time lit) can be caused by these same things.

You need a code scanner.  Inexpensive ones like the ELM327 clones on Amazon or eBay work fine for this, along with a phone and the free version of "Torque."  If you have a Ford or Mazda Forscan on a PC is free for most diagnostic uses and a cable version of the interface is about $30 -- and gives you access to much better diagnostics than you could otherwise obtain with the inexpensive tools.

However -- if you own multiple vehicles from various makes then I strongly suggest you consider the OBDLink MX+.

Its not the cheapest option but is one of the best outside of professional-level tools such as the Autel series (and beware, some of the Autels are extremely good and have configuration and coding capabilities but they're not cheap and the cheaper ones are not nearly as capable.)  In general if you need coding capability, that is the ability to configure rather than just diagnose, then outside of Mazda and Fords, where Forscan may do what you need without having to fork up money the options all involve funds and most involve subscriptions of some sort.  I won't go into those except to say that if the car is a VWAG product there's only one to consider and that's VCDS from Ross-Tech.  Other than key-matching (which is a function of VW not letting the required security code "out" for your particular VIN) it can do virtually anything the dealer can.

So why spend the extra money if you have other than a VW and are wondering "why the $140 tool rather than the $40 one?"

Mostly because the OBDLinkMX+ comes with an app that can read the other modules on virtually all US-market cars beyond the engine (besides also working with Forscan if you have a Ford or Mazda assuming your laptop has bluetooth.)  So if you have, for example, a temperature knob that isn't working on your car, and the blend doors are electric on your vehicle (they are on many) it can read that module too and tell you which of the actuators is bad.  As an example of this a friend of mine had a dash light on recently on a newer Jeep that has "intermittent start/stop" (the engine stops at red lights, etc.) and this scanner was immediately able to determine that the fault was that the car believed the small, secondary battery was bad (as the wiring looked ok best bet would be to go get another battery and replace it and it'll be fine.)  That app is included, fairly-frequently updated and thus is very useful across multiple makes and models at modest cost -- and will probably work with whatever you buy next.

Ok, so back to the "generic" stuff.

The first thing to understand is that all gas engines and their computers work essentially the same way.  When the engine starts it has a "base" map in it that tells it how much fuel it can inject for a given amount of airflow, and it knows the air pressure in the manifold and how much is flowing from two sensors (the MAP and MAF, respectively.)  Until the oxygen sensors start working, which requires that they get hot, it does not know how close to correct it is -- it only knows a "safe" (but a bit rich expected mixture) amount of fuel for a given amount of air.

As soon as the oxygen sensor(s) get hot then they tell the computer whether the mixture is lean, rich, or spot-on.  This is called closed-loop operation and is how your car is supposed to run all the time.  When it is running "closed" it is burning extremely clean to start with, and the catalyst(s) can then consume the rest of the extra fuel and produce very, very low emissions.  In fact modern engines with catalysts, once warm, produce so little carbon monoxide that it actually requires some effort (or extraordinarily-bad luck) to poison yourself -- this was certainly not true in the days before those systems when every car stunk (and spewed out a lot of CO) as it went down the road.

The computer knows that the sensors are working because it continually cycles the fuel amount up and down a tiny bit and looks for very small changes on the oxygen sensor output that match what it is doing.  This allows it to know the sensor is not broken (or that you haven't tampered with it by simply providing a fixed voltage output) and thus that it really is measuring what the engine is doing and that the engine's fuel/air mixture is not only good from an environmental standpoint but that it is safe for the engine to run as well.  Additionally the catalyst requires something to burn up in order to remain hot and functioning normally so a bit of rich mixture followed by a bit of lean (extra air) keeps it operating properly without adding to emissions.  In addition there are one or more knock sensors that tell the computer if detonation occurs -- which happens if the octane is not high enough, ignition timing is too advanced and under some load conditions.  That sensor is basically a pizeoelectric element that resonates at the frequency where gasoline ignition knock is centered, and if it generates a signal then the engine is knocking and the computer knows to back off.

Most of the time if the light is on the code will tell you what is going on and is pretty straight-forward -- but there are exceptions and some of them are used by unscrupulous mechanics to screw you.  So use your reader, get the code, and then use Google (if your software doesn't tell you directly) to find out what it means.

Here are some examples that are often found and can be abused by mechanics:

Cylinder-specific misfire codes (typically P0301 - P0312.)  These indicate that one cylinder is not firing correctly.  This can be a single injector, a single ignition coil, the wire between it and the plug (if there is one) or a single plug.  Older engine computers are not very sensitive to this and thus only pick up really bad misfires but the more-modern engines are extremely sensitive.  Usually by the time you get a check engine light for this you can fell that its running a bit rough or worse.  The injector is hard for you as an ordinary person to check but the coil pack, wire if any and plug are not.  If you remove the plug and check both its condition and the gap (a feeler gauge is inexpensive) and both are ok exchanging the coil pack and (if there is one) the plug wire with the cylinder next to it will either cause the misfire to move or it will not.  If it does move then the coil and/or wire is presumably bad.  Many mechanics will try to sell you all new coils, any wires involved and plugs at once when this happens but it is extremely rare for all of the coils o wires to fail at once and swapping a coil is a few minute thing.  Note you need a torque wrench for the plug as all modern heads are aluminum and can be stripped, and NEVER use anything other than your FINGERS to thread a plug in all the way into the hole; if you cross-thread the cylinder head it typically has to come off to be fixed, you can't run the engine until it is fixed and that is very expensive!

If you have a misfire in one cylinder, you change the plug (or its ok) and move the coil and it doesn't move then the injector is the likely cause of the fault.  This is a bit more-involved and for many people you will want to obtain help for that -- often the fuel rail has to come off to replace one injector, many times all the seals have to be replaced and leaking fuel is a serious fire hazard so that's a "second level" problem. But checking a spark plug and moving a coil pack is not and often is a 10 minute deal.

Random misfire (P0300) is as the name implies random and not located in just one cylinder.  This is often fuel related (e.g. bad fuel pump or similar) especially if it happens when the engine is under load.  Diagnostics start with fuel rail pressure and again here you might want professional help as again fuel leaks are of course very bad.

Catalyst efficiency codes (P0420/0430) are one where people often get rooked.  The code implies the catalyst is bad -- and they're expensive.  V engines typically have two, one per cylinder bank, while inline engines usually have one.  Note that on modern cars there is often a secondary, large catalyst under the floor -- that one, if present, is not monitored at all.  The one its complaining about is right up near where the exhaust exits the engine and there are two oxygen sensors, one on each side of the catalyst.  (Some cars, notably some Ford engines, have a "pre-cat" in the exhaust manifold -- this too has no monitoring and is a bad design -- if possible avoid those cars as if that breaks up pieces of it can wind up back into the engine and damage or destroy it!)

Remember up above I said that the computer will oscillate the fuel mixture deliberately above and below threshold.  The catalyst code comes up if the second sensor, after the cat, does not respond as expected -- it should either slowly ramp up and down in output or, under somewhat higher load conditions (e.g. steady driving on the highway) have a steady output voltage around 0.8-0.9v.

The problem is that things other than the catalyst being bad can cause this reading to be wrong.  The sensor can be bad and is about $75 instead of $500 or more for the catalyst, for one.

But before chasing any of this you want to look at fuel trims.

What are fuel trims?

Simply put they're the "offset" between what the computer thinks it needs to inject for fuel and what the sensors tell it that it must actually inject.  There are two per bank on the engine -- short-term and long-term.

The two are added together and "spot on with what I expect" is 0.0.  If the trims are negative then the engine actually requires less fuel than the computer thinks it should; if they're positive then it requires more.  The sensors and individual engines are never exactly on with zero error so anything +/- 10 is reasonable and expected, although most engines run around +/- 5 under normal conditions.

Anything beyond +/- 10 is abnormal.  However, the light will typically not come on until +/- 20-25.

If you get one of these codes (P017x) and the light is on throwing parts at the problem is a mistake.

Here's how you interpret it:

1. If the trims are negative then that means the computer is being forced to cut fuel.  This implies that either (1) the MAF (mass airflow sensor) is bad, (2) the MAP is bad (its reading less pressure than is really there) or the injectors are leaking.  If the negative trims are worse at idle and/or lower load but are more-normal at higher output (e.g. while accelerating or climbing a hill) this implicates the injectors because a leaking injector usually leaks the same amount but the amount required to run the engine goes down, so proportionally the leak is worse.  If its even across the board then the sensors are more-likely to be the cause.  This one can be fun to find without throwing money at it; the MAP (pressure sensor) is easy to test in that with the engine off but key on it should read atmospheric pressure (14.7 psi at seal level) and at idle the throttle is closed, vacuum is at maximum and it typically will read 5-6 psi or thereabouts.  If its wildly outside of that it is probably bad.  MAF is tougher; there is a code for it being bad but that is notorious for not being set even when the MAF deteriorates to the point that fuel economy and/or performance is severely impacted.  Do not drive the vehicle with serious negative fuel trims as it is very likely you are also getting significant oil dilution if one or more injectors is leaking and that can quite-rapidly ruin bearings or the cylinder walls of the engine.  If you have an oil pressure gauge and it is reading lower than normal this is almost-certainly occurring.  Find the problem and fix it immediately.  Fuel dilution and leaking injectors are especially-serious in more-modern, high pressure fuel systems.  Trims that become more negative with engine speed increasing can also imply exhaust restrictions.

2. If the trims are positive then the computer is injecting more fuel than it thinks it should need to.  This again can implicate the MAF or MAP -- and also possible fuel delivery problems -- but it also implicates intake leaks.  If the trims are strongly positive at idle but more normal or even normal under load (when vacuum is less) this indicates a leak in the intake after the throttle plate because at lower vacuum in the intake it leaks less.  A stuck PCV valve or leaking intake gaskets and similar can also cause this.  If the positive trims are greater at idle than under load it is almost-never the the actual throttle body or the fuel system that is involved -- many mechanics will get this wrong.  A more-strong positive trim under load and normal trims at idle, on the other hand, implicates fuel delivery problems or leaks after the MAF but before the throttle body.  Any air leak after the air filter needs to be corrected as it is allowing unfiltered air into the engine and if not fixed will severely impact engine life.  The car is safe to drive as lean mixtures at idle, not under load, will not usually do any damage but the unfiltered air over time will.

3. Only if the sensors before the catalyst are in range, along with trims, should you believe the catalyst is actually bad without solid evidence and that you have an efficiency code is not enough.  This is not to say they don't fail (they do) but an awful lot of people buy them when they're not broken.  Other than by poisoning (e.g. running leaded fuel, which is sort of tough to get these days unless you go buy 100LL avgas!) or running for a long time with wildly out-of-range fuel mixture they typically do not get destroyed or break up and cause restriction in the exhaust flow.  If you have an efficiency code and the trims are normal, and both sensors have been changed (you're going to have to buy new ones with a new catalyst anyway if you have any sense at all) can you reasonably believe that is actually bad.

4. Note that exhaust leaks before the cat, if severe enough, can result in trims being out sufficient to trip these codes or a catalyst code as well. Typically, however, if the exhaust is leaking badly enough at the manifold or the pipe between the manifold and catalyst to produce that sort of code its really obvious (obnoxiously loud and stinky.)

These are some of the most-common problems that show up -- and a good part of the time the fixes are inexpensive and easy.

No matter how complex the other systems are the common point is that all modern fuel-injected, catalyst-equipped engines that run on gasoline operate almost-exactly the same way.  There are differences of course between makes and modes but they are in the finer points and not the basics of how the computer keeps the engine running both safely and with minimum emissions; the foundation of that technology goes back to the 1970s although modern port-injected engines started to show up in the 1980s.  Anything 1996 and newer in the US has OBD-II and thus is scannable and can be interpreted in this fashion.

Got something specific?  Post it in the comments and I'll tell you what I know about it.

Let's go through a few things here that might save you a TON of money....

First, vehicles today are far more complex in that they have all manner of electronic doo-dads and fancy stuff on them.  You might think from this that they really are more-complicated in what makes them move and stop.  For the most part you're wrong.  There are more systems but, at least for gas-powered vehicles (diesels are different) the basics of how the vehicle actually runs and regulates itself has not changed much since the first fuel injection systems.

All of them work basically the same way:

When the engine is cold (before the catalyst and oxygen sensors are functional; they must be hot to work) there is a "base fuel table" in the computer.  For this much requested throttle at this pressure (altitude) and this RPM I can inject this much fuel and we have stable, safe combustion.

This is always a bit rich because being lean means detonation and, if severe, holes in pistons and other very bad things.  Old-timers who remember carbs know that they are always set slightly rich for this reason (which is why they stink too, by the way; that's unburned fuel.)

Modern engines work real hard to get those oxygen sensors online, which means hot.  The strategies differ but often include both an electrical heating element in the sensor and manipulating the valve timing in the engine (Mazda in particular is known for that in the Gen3s) and idle so as to get it hot faster.  Why?  Because once the oxygen sensors are working the computer does not have to guess.

The sensor in front of the catalyst tells the computer if the engine is running rich, lean, or right up the middle.  The computer deliberately varies the mixture very slightly from rich to lean -- just a tiny bit -- to prove that (1) the sensor is working (and you didn't trick it) and thus (2) the engine is operating safely.  The second article in this series will explain how to look at that and figure out some of the reasons that "check engine" light is on -- it is telling you the computer doesn't like something it sees when it lights up.

But this article is about routine maintenance and inspections.  You do them.  No, you don't pay people to do them.  Why not?  Because they might do it wrong and I guarantee what in the manufacturer's book is not bad, but it also probably isn't enough, and further, you're going to get wallet-raped if you go there.  If you have to then you do, but if you have a garage you should do these things yourself because then you know they're done right.

Note that every bolt on that engine has a torque specification.  Yes, you need a torque wrench for some of this but you can google nearly anything now so you probably don't need a service manual.  Good thing too because most of the time nowdays you can't buy a factory one for reasonable money; they all want subscription fees.  It used to be that $20-50 would buy you the shop manual.  Not anymore.  Note that the "Clymers" and "Haynes" are trash.  Yes, I thought they were ok too long ago -- they're not.

So here we are with the list of things you should be doing.

First is oil and filter changes.  The manufacturer provides two limits -- "ordinary" and "severe" service.  Virtually everyone is severe.  What is severe?  Short trips without fully warming up (oh you don't do that 3 mile run to the grocery store, do you?), lots of hot idling (you don't sit in traffic in summer with the A/C blasting, right?) and similar.  You probably do some or all of that.  So follow the severe schedule.  Change the oil and filter on schedule, period, and do not use "Jiffy Lube" and similar as they all use whatever mass-produced filter is cheapest and often bulk oil which would be ok EXCEPT it is almost-never fully synthetic.  Either buy ramps and wrenches and do it yourself or take it somewhere that will use quality product.

Use either factory filters or the NAPA Gold or Platinum series.  Most of the rest are relabeled from someone else.  Trust me on this and no, the so-called "performance" filters are not necessarily better but they are more expensive.  I like the NAPA Platinum for GM and Fords but on the Mazda I use the OEM ones because the dealer in Ft Walton Beach sells them six for the price of five, so they're both of good quality and reasonably priced.  Note that it is not just "how well" it filters; it also is a function of how well the filter flows oil as all the oil that lubricates your engine has to go through the filter.  Too restrictive is bad which means there is a balance.  The reason NAPA Platinums are good for GMs in particular is that they are larger-capacity (less pressure drop) and reasonably priced and the media in them is excellent and they will fit without sticking out where they can be damaged.  That makes them a good choice.  The OE is also and always an acceptable choice.  Spare me the BS about "super filters" from this manufacturer or that -- my Mazda has 250,000 miles on the clock, it has never had other than an OE filter on it, I change the oil myself and have since the factory fill and it consumes less than a quart across its entire service interval with no lubrication-related issues or wear.  There is no "better" than that so spare me the BS and added expense.

Follow the factory oil specification.  You do not know better than the manufacturer does.  Use a full-synthetic; pick a brand, it matters very little.  Even WalMart's "SuperTech" synthetic oil is of good quality provided it meets the specs.  The most-important thing is the weight (e.g. 0w20, 5w30, etc) but always buy the synthetic, not the synthetic blend.  The reason is that engines made after about 2000 all have the top compression ring very close to the fire deck -- that is, right at the top of the piston where the combustion happens.  This is done because it reduces the amount of "dead space" where fuel and air can collect but not burn and is required to meet emissions specifications.  That also means the oil runs hotter, and if you use old-style mineral-based oils you run the risk of it burning in the grooves, sludging up the engine and doing severe damage.  Even if the manufacturer doesn't require synthetic use it anyway; it's another couple of bucks and you're crazy not to.

Be aware that some manufacturers have specific requirements that are not part of the service class (e.g. SN and the weight.)  This is most-common with diesels but not unique to them.  VW diesels in particular are known for this and using oil that does not meet their specs can result in severe damage, including destroyed camshafts and emissions equipment.  Read the owner's manual - yes, it IS in there.  In general gas engines with a "Sx" requirement (e.g. SN, etc.) are ok with any later last letter but this is not necessarily true for diesels.  Again, if you own a diesel pay attention to the requirements in the owner's manual.

When you get a vehicle check the oil level at every fuel fill until you have a very good idea how rapidly it consumes oil.  All engines consume a little bit of it but some consume more than others.  Too much or too little oil is very bad and can result in immediate and severe damage.  Running too low and having low pressure as a result will destroy an engine inside of 30 seconds if its under load (e.g. you're using it to move the car!) and most modern cars do not have gauges so you get no warning as by the time the oil light comes on you're frequently screwed.  Too much oil will cause it to be above the level of the crankshaft which will "whip" it and can cause low oil pressure and serious damage up to and including engine failure.  Many engines will use more than the one quart between "full" and "low" between change intervals so you cannot just change the oil and forget about it without the risk of severe engine wear or even destroying the engine.  Once you know your car's pattern then you can relax a bit but until you do check every time you put fuel in and if in doubt pull the stick when you are fueling and check.  If you have an engine that has oddball oil requirements or your car typically uses a quart or more between changes buy an extra quart and put it in the trunk so you are not forced to put something in there you shouldn't if you're on a trip and find its low.

Related to this if you have a turbocharger on a gas engine never turn the engine off immediately after running it hard, including cruising on the highway.  The turbocharger is cooled in part by the oil flowing through the center of it in a labyrinthine path -- because of the extremely high speed that it rotates at (100,000 RPM+) it does not have traditional contact bearings.  When you turn the car off the oil flow ceases immediately and the exhaust gas going through it can exceed 1,000F.  If the turbo is hot when you shut it off the oil that is in there can cook into carbon and once that starts the damage is progressive.  Always let the car idle for a couple of minutes before shutting it off after a hard run -- including when pulling into a highway rest area.  This is less-true for diesels, incidentally, because a diesel has no throttle plate so when you take your foot off the accelerator the engine continues to pump all the air through the turbo (but no fuel) and cools it quite-rapidly, where with a gas engine there is very little airflow because the throttle plate is closed.  Nonetheless its a good idea to let a turbodiesel cool down a bit as well.  There are plenty of people who claim "turbos blow up a lot."  They do if you abuse them.  The TDI Jetta that Sarah owns has nearly 300,000 miles on it -- and still has the factory turbocharger in it.  I've owned several others, including two turbo Volvos (gas engines) and have never had a turbo fail.  The reason is because I NEVER shut them down hot.

Also, and related to this on a cold start let the engine idle for 30 seconds or so before taking off and accelerate gently until normal operating temperature is reached.  Most wear occurs in the few seconds at cold start on an engine; immediately throwing the car into gear and taking off at high speed, putting a lot of load on it, just makes it worse.

After oil the next most-important thing is air quality.  This means the air filter.   GM ran a study a long time ago and showed that virtually all engines wear out not because of time or oil changes but because they ingest almost exactly one teaspoon of dirt.  Now that can take five minutes or it can take 500,000 miles but however long it takes that is how long the engine lasts!  No, the fancy-pants air filters and "cold air intakes" sold by various aftermarket companies are not better; in fact they are almost-all much worse, often by a factor of 10, 100 or more!  Specifically, do not ever use 1970s tech level stuff like the oiled reusable or "cone" filters!!  They are TRASH when it comes to actually filtering out dirt compared with modern synthetic/paper style elements.  Pay the money for the quality product but don't believe the BS about having to change it the quick-lube places will run on you.  If you remove the element, hold it up to the sun and can see light through it its fine -- put it back in and run it.  How long it lasts depends on how dusty your area is.  Some engines have a restriction indicator and as long as its in the green you're fine.  Most drivers will only need to change the air filter every 30-50,000 miles but driving through one dust storm will trash it within minutes so there is no rule that works for everyone.  Check the air filter with each oil change.

Along with this it is absolutely essential that the clamps and hoses between the air filter and the intake are in good condition.  Missing or loose clamps -- or cracked hoses -- will not just cause problems with engine's operation it will also dramatically shorten engine life because unfiltered air can get in through those holes.  Inspect this every few months carefully and replace anything that has deteriorated.

Next up is the cooling system.  Modern engines have aluminum cylinder heads -- many are cast-iron blocks but some of those are now sleeved aluminum as well.  That means the metals in the engine expand at different rates because they're different.  Overheats will often crack or warp the head(s) which is a $1,000+ repair and if not caught immediately can destroy the engine through oil contamination by the coolant; there is no longer such a thing as a "mild" overheat on a modern engine.  The cooling system has a thermostat in it that is designed to keep the temperature of the coolant -- and thus the engine -- within a very narrow range  Most modern engines have 187F thermostats which means they start to open at that temperature and are fully open at 100C (212F.)  If your engine is slow to warm up the thermostat is sticking open and that is bad for both the engine and fuel economy.  They're designed to fail open (although they can stick closed) so if you detect slow warm-up that is likely the cause.  On most engines they're easy to replace and reasonably-inexpensive but again buy quality parts here as Chinese garbage is all over the place and the last thing you need is one that doesn't work right.

Most newer cars come with a long-life coolant.  These are typically good for 5 years or 100,000 miles from the factory and you should never put "universal" or "parts store" coolant in these systems.  Mixing chemistries, even if only a few ounces, often turns the coolant to brown sludge which does severe damage, and there are several different chemistries from different manufacturers!  Second, never put tap water (or mix with tap water) into a modern system.  Only deionized or distilled water is safe and only with the correct factory-specified coolant in the correct proportion, or the proper 50/50 premix.  The coolant should not "disappear"; no engine normally consumes it. If your coolant is low it is either leaking somewhere (bad) or being burned (much worse.)  If your car is losing coolant figure out why and fix it immediately.

There are two types of cooling systems -- open/recovery and closed.  Manufacturers use both so you cannot assume -- you have to look.  If you have a recovery bottle that has no pressure on it (e.g. the Gen3 Mazdas, Sarah's Express 1500 van and many others) then the system is open; that is, the radiator cap regulates pressure (typically to 15psi), the radiator is filled completely to the top, and when the coolant expands as it gets hot it forces some into the recovery bottle which has a suction tube at the bottom with the top open to the atmosphere, usually with an overflow hose directing any overflow safely just in case.  When the engine cools it sucks the expanded coolant back in.  This is fine but it means new oxygen from the air can get to the coolant and a small amount of it will each time the engine gets hot and then cools down again -- and degrade it.  Closed systems are used on some models (e.g. my GM Suburban and most VW/Audi products, for example) which have a "ball" or "reservoir" that has an airspace at the top and the pressure cap is on top of that; there is no separate cap on the radiator.  These systems must not be filled beyond their "full cold" line as the airspace is designed to compress when the engine warms up and in doing so prohibits air exchange.  This is a superior design decision, incidentally.

Never take the cap off a cooling system when the engine is hot.  It is under pressure and the coolant is not boiling because of that pressure; if you remove the cap all the coolant will immediately flash boil and said boiling coolant will blast out on everything around the engine, including you.  In addition the vapor from coolant is in fact flammable so in a severe overheat its possible to get a fire if you do that on top of being severely scalded.  Yeah, don't do that.  Wait for the engine to cool down to the point you can put your hands on it without being burned and then slowly turn the cap either to the first stop or, if it doesn't have one, using heavy gloves very slowly remove it.

Again never mix coolant types and never use so-called "universal" coolants.  It is ok to buy, for example, Prestone's "GM" formula as it is a Dex-Cool formula (as is the OEM AC Delco stuff.)  But never buy a universal or go by "its the right color" for replacement.  Ever.  3rd Gen Mazdas and modern Fords run FL-22, which is tough to get in the aftermarket (Ravenol used to have it available as a concentrate but it disappeared a couple of years ago) so getting reamed to a modest extent by the dealer is something you really can't avoid.  VW has their own specific stuff and so on.  Buy the factory-specified stuff.

If you have a closed cooling system then after the first five-year drain and fill (NOT a full flush which incidentally is impractical as you can't get it all out and if you use tap water to flush you will screw up modern engines -- so don't unless you are prepared to use concentrate and a LOT of distilled water for flushing!) you can run a second five year interval provided the coolant continues to look clean and of the same color as it came out of the bottle.  With open systems after the first change you must dump whatever comes out of the radiator and refill it every 2-3 years or 50,000 miles, no exceptions, or else.  Again, overheats kill engines and in addition corrosion kills heater cores which frequently requires taking the entire dashboard out of the car to replace it, so if you like $1,000 repair bills or worse, a cracked cylinder head and destruction of the engine then go cheap on a $20 bottle of coolant and an hour of your time.  What's even worse is that on an automatic transmission vehicle the transmission cooler is in the radiator and so if you don't maintain the coolant it can corrode through and destroy a $5,000+ transmission by pumping coolant into it when you turn the car off.  Toyotas are known for this and yeah, that's what can happen if you neglect it.  Check it when you change your oil and if the coolant is dark or has any sign of degradation change it.  A bottle of coolant is cheap and most modern vehicles have a drain cock at the bottom of the radiator that makes doing so easy.  Note that you need to pay attention when you refill and first run it -- typically with the cap off on the first start and the heater set to "full hot", making up any shortage -- and watch the temperature gauge just in case you get air in the system in a way that doesn't clear.  At any sign of trouble until the system is known full turn the engine off and figure out what's going on.

Brakes.  European models typically specify DOT 4 fluid and US and Japanese models typically specify DOT 3.  Now here's the rub: European vehicles typically specify replacement of the brake fluid every two years.  US and Japanese cars typically have no service interval at all.  A quart bottle of brake fluid is literally under $10 and if you have a power bleeder and a turkey baster (or large syringe and a bit of tubing to suck the old fluid out of the master) changing it requires about 30 minutes including running the new fluid through to all four wheels.  If you do this you'll never buy a master cylinder, ABS pump or caliper and done correctly you do not need a scan tool to get air out of the ABS pump either.  It is foolish beyond words to not change the fluid in any vehicle you intend to keep beyond 4-5 years because ABS pumps are expensive and master cylinders and calipers can be a pain in the neck to change and bleed, especially with modern vehicles that all have ABS.  At the same time you do this job you can (and should) check the seals on the caliper pins; if there is evidence of damage repacking them takes minutes and will keep you from prematurely destroying a set of brake pads and a rotor.  You have to take the wheel off to do the fluid bleed so this is easy.  Would you like to spend $10 or $1,000+ on a new ABS pump (and many of them are a pain in the neck to get to and swap as well -- GM trucks, for example, have them under the vehicle and access, while possible, is not a lot of fun.)  At the same time this also means you can get a quick look at the brake pads and assess their wear.

Belts.  Examine on every oil change.  If you see cracking on the inside of the ribs the belt is toast.  Many vehicles have an indicator on the tensioner that will tell you if the belt is stretched too far and should be replaced, but if not a good interval is five years or 100,000 miles assuming you see no evidence of trouble on the belt ribs.  A broken belt that drives the water pump will cause an immediate overheat and even if the water pump is not driven by that you will lose A/C, charging and possibly power steering.  Some are nearly impossible to change on the side of the road too, so this is something to look at regularly.  An immediate tow requirement is not cheap and of course on vacation is especially annoying.

Transmission.  Manuals should have their oil changed every 50,000 miles, automatics changed every 30,000 and the pan dropped and filter changed on the first service and then every second one thereafter.  NEVER use a flush machine or let anyone else do so.  The issue is the same as with coolant; the fluids are incompatible with each other and there is no way to completely clean the machine between uses so if the mechanic screws up it can destroy your transmission shortly thereafter.  Don't do it.  Automatic gearboxes will only drain a third to a half of their fluid because the rest is in the torque converter and doesn't drain so do the service on that interval and once again use modern, full-synthetic fluids where they're approved even if not what was originally in there.  This again is where you want to use the approved factory fluid and no, all those fancy-pants really-expensive so-called "better" fluids aren't a good idea and probably isn't better.  Again many people never change this fluid and modern car manufacturers tend to claim the fluid is a "lifetime" fill.  It is: For the life of the transmission which will likely be 100,000 miles or so if you never change it at all!  Transmissions are expensive, especially modern ones.  Note that with very few exceptions manual transmissions specify GL-4 fluid.  Never put a fluid that has GL-5 on the label in such a gearbox even if it also says GL-4; you may destroy the synchronizers if you do within 30-50,000 miles and they're a bitch (read: $$$$) to have changed as the transmission has to be completely disassembled to get to them.

These are the big ones.  There's more of course but if you do these a huge number of potential "gotchas" will probably not bite you.

BTW while I do very much like older (pre-DPF and DEF systems) diesels in passenger cars and light trucks modern ones are a serious and expensive pain in the ass.  All of them have a DPF in them which is integrated into the catalyst and it will fail from simply wearing out and require replacement, typically around 100-150k miles -- and because the catalyst is in the same physical case and the catalyst is full of rare earth elements it is expensive.  In addition they all have a high-pressure fuel pump that, if it fails, will almost always require replacement of the entire fuel system including the injectors, fuel lines and even the tank because it will spray metal shavings all over them and the failure is usually zero-notice too so of course its likely to happen when you're on a nice, long trip 1,000 miles from home.  This is usually a $5,000+ repair with $3k+ for the parts as is the case with the DPF!  In short if you buy one of these you're purchasing a couple of planned $5,000+ repairs every 100,000 miles or so which, from my point of view makes them ridiculously uneconomic to own.

Good motoring and the next article in this series will explain how to read a scan tool and figure out a lot of the common codes you might see. Most are actually not expensive to fix and knowing what is going on, even if you have to hire out the work, is an excellent defense against getting robbed by unscrupulous or lazy mechanics who simply swap parts rather than figure out what is actually wrong.

.... the gathering gloom.

Plenty of people -- including obviously the stock market and its commentators -- believe everything will be ok.

Deficits don't matter.

The government cannot go broke.

There will never be a loss of confidence.

I travel quite a bit.  Like most people I have my habits -- places I like, things I go to do and if I enjoy them I'll go back and do them again.  This means I see patterns and, unlike many, I tend to notice them immediately.  Perhaps its a blessing -- or perhaps a curse.

I just got back after one such trip; one I've made before and Sarah and her boyfriend both came as well (her boyfriend has not been on this trip before, but she certainly has.)  They traveled separately but we met up and had a good time.

If you're in the market you ought to be making up a big fat list of things to be short -- or at least be out of the things you've had and are long of, particularly when you can get a 5%+ risk free return in the short end (the 13 week bill, for example) of the Treasury market.  I was stunned at the deterioration I saw in consumer behavior from just a month or so ago on my prior trip, and gob-smacked at the change over three months or so back when Sarah and I were out at Wolf Creek, also a place I like to go on a repetitive basis (for skiing, of course.)

There was evidence of it then on our travels which were over a large part of the same path we took back in September for our trip out to the Grand Canyon.  That which was jammed full was not, and yet there wasn't a serious price delta between those two trips; that is, while there had been lots of inflation over the previous couple of years (and it was obvious) there wasn't anything that was a sort-of "trigger moment" associated with the change, and being winter .vs. late summer, ok, maybe it was seasonal and frankly, the delta was small.

This time it wasn't small and in addition there were serious price hikes that were attempted -- and appear to have had an instant impact.

I'm talking about sit-down fast-casual places I like when traveling and lots of others do too in that they're always slammed to the point that if you want to eat there during the dinner hours I hope you like their "get a sort of reservation on their app" thing or sit at the bar if its just one of you -- because if not you might be waiting an hour.

Well, that was gone.  And not a little gone either.  The place had plenty of tables with no wait and the parking lot had lots of spaces too.  But it wasn't one place -- on the way home I stopped at another I've been in a dozen times over the last five or so years, again, at dinner time, and a third of the tables were empty.

But what else was shocking?  A roughly 40% total price increase over a couple of years ago and of that 10+% that just got tacked on with all the menus having just been reprinted in the last couple of weeks.  It appears that last grab for cash finally hit people's pain points and they stopped showing up.

I also saw "fast food" style places co-located with fueling stations on the highway shuttered and boarded up -- and that's entirely new, and of other chains I pay some attention to during the same times in the afternoon and evening their parking lots are half-full or less.

Yes, this is "shoulder season" -- or is it?  Not really.  Its graduation season and a lot of people are in fact traveling for precisely that reason; in another few weeks it will be the start of "traditional summer" with Memorial Day.

Folks, there are no rate cuts coming because there can't be into this government spending level without an explosion in inflation.  Without taking on the place in the federal spending game where all the money is going (that's health care) there is no fix.  Attempting to work around it with more offshoring, more robots, more data centers (and "ai" in them) and similar won't work either because you can't print money -- you can only print credit and to obtain money someone has to do something of value for someone else in excess of its costs.

When costs ramp that excess closes and eventually goes negative -- and at the point it does that activity becomes uneconomic.  If you continue to do it through various machinations such as the government playing handout games you run the risk of an exponential runaway and collapse.

But what you should keep in mind is that never in reasonably-modern history do markets let you get to that endpoint.  They didn't in 2000 or in 2008 either.  All the hollering about "subprime being contained" proved to be nonsense; the underlying bubble that "supported" the charade was seen through before the endpoint and the market imposed its own view of things whether policy makers liked it or not.

Mature fast casual dining and chip companies selling for 70x earnings are fantasy-land nonsense and yet both are the case right here, right now.

I get it that nobody likes the implications of prices having to collapse by a third to come roughly into line with incomes, but its fact.  Further its at least double that in the capital markets because common stock always has an element of leverage in it (otherwise why would it sell at a "multiple" of earnings at all -- and yet it always does, does it not?)

The belief that The Fed "must" or "will" step in and prevent such a reversion to the mean is absurdly common -- after all, they have stepped in through the last two decades (or even more) but in doing so each time they've made the imbalance worse and now the exponential nature of such deficit spending and debt load are here rather than a future problem.

For those who believe that it will "never fail" or worse, that you'll get plenty of warning before something serious breaks I have three words for you:

SOLD TO YOU.

In short if you expect rental income flows to remain stable and yet you also think valuations should go up rather than down you're dead wrong.

Here's the simple fact of it: "Cap rate" expresses the return on invested capital as a percentage.  A $500,000 property that has net operating income (that is, income minus operating expenses) of $25,000 has a "Cap Rate" of 5%.

What is a reasonable "cap rate"?  It is always higher than the short-term interest rate on Treasuries because there is risk in a real estate transaction but there is no risk in short-term Treasuries.  That is, if I get a 5% payout on $500,000 worth of 13 week T-bills rolled over every 13 weeks for a year (that is, the IRX is at 5%) for a real estate transaction to make sense it must pay materially higher than that cap rate because I cannot control for the risk that the value of the property may decline, there is significant expense in disposing of the property if I choose to do so and real estate is illiquid (that is, I cannot call someone or get online and get rid of it in a day or three.)

My view has always been that a reasonable minimum cap rate is around 7-8% and I am personally not interested in anything under roughly 10% in real estate because the illiquidity is always there yet it is frequently ignored in terms of the risk profile yet it is in fact the most-extreme problem since it can and does come up at the worst possible and can prevent an exit for six months or more!

There are two ways for cap rate to go up -- the rental can increase or the price of the property can go down.  Of course if the latter happens and I already own it I'm the one who is now sitting on a large capital loss -- that's bad.  If someone else is sitting on it and I want to buy it that's good.

My general rule for any business transaction is that you don't make money when you sell things -- you always make money when you buy them.  This sounds backward but it is not; buying at a good price is always preferrable to trying to figure out how to claw your way out of the hole when you go to sell whatever that thing might be.

People have gotten seriously-intoxicated on the formerly-suppressed rate environment, especially during the 2020-2023 time period.  Those days are gone and they're very unlikely to come back in my remaining lifetime.  If you expect to see that again within a decade or two I believe you're going to be disappointed at best and are quite-likely to be ruined financially, especially if you are in any way reliant on that.

Buying property to live on is not the same thing as buying property for commercial purpose, whether long-term or short rental or, for that matter, to set up a business on the land itself.  Those are very different calculations and one has nothing to do with the other; a home is a consumer durable good, not an investment despite the claims otherwise by many over the last couple of decades.  There are a huge number of external factors that can ruin it from an investment perspective (e.g. property taxes shooting the moon) which happen often enough over long periods of time that counting on such is a very bad idea.

Never mind that the "30 year mortgage" never made sense in a world where the average holding time is seven years -- and that latter has been true for decades.  Why?  Go look at an amortization table; you only pay down 10% of the principal during that seven years.  All of the rest of the money went to the bank!  It was the insane view over the last two+ decades of price appreciation that made people think this "worked" when in fact it never did because permanent price appreciation in excess of actual realized inflation is mathematically impossible -- the only question is when that will end, not if it will.  But the securitizers and security holders (e.g. MBS) all banked on that same seven year duration and now, with rates much higher all the older securitized loans are not prepaying (I wouldn't either if I had one when I have a 3% mortgage but can get 5% in short-term Treasuries with no risk!) so now the worm has turned against said holders.  Oops.

In the investment world, including both longer-term "conventional" rental properties and especially the short-term rental world, along with commercial real estate, all this applies and the shorter the term of the rental the greater the risk and thus the greater the cap rate you should and skilled persons will expect because those who overpaid and want to or must sell are sitting on huge capital losses!

Right now there is a severe imbalance in this regard in that cap rates are in many cases below the short-term Treasury rate.

IMHO you'd have to be out of your mind to accept that sort of alleged "return" and thus, at those prices, to be anywhere near that space.