Your car is trying to tell you something!
It's the sight car owners dread - the glowing "check engine" light on the dash flickers to life, though in many cases there doesn't even appear to be anything wrong! When there are no odd noises, no rough idle, and no flames shooting out from under the hood, reactions to this light seem to fall into two categories: 1) Ignore it, and 2) Run to the dealer to find out what's wrong.
Option 1 is a bad idea, because, quite frankly, the increasingly sophisticated on-board s systems in today's automobiles are probably better at spotting trouble in the making than you are. Yes, there are anecdotal stories of people driving around for years with a flashing Check Engine Light... but I wouldn't recommend it. Ignoring what is likely a very simple problem now is a good path toward bigger, more expensive repairs in the future.
Option 2 is better, but has its own drawbacks. Your dealer will probably at the very least charge you an hour's labor just to plug into your car's computer and retrieve a code that tells them where the problem lies. And since, let's face it, the auto repair industry doesn't exactly have a reputation for scrupulous honesty, you're left vulnerable to those who might read a code that suggests a faulty spark plug, but tell you major engine work is required instead.
So let's introduce Option 3, the do-it-yourself option. No, I'm not suggesting everyone become a full-blown auto mechanic; I'm suggesting that every car owner should be aware of the inexpensive tools and free resources available to avoid those diagnostic fees, potentially make smaller/easier repairs themselves, and in cases where a trip to the shop is necessary, to be able to approach it with some information on their side to help protect them from fraud or bad diagnoses.
But first, a brief history lesson
Electronic on-board automobile diagnostics have been around since 1969 when Volkswagon introduced them on one of its fuel-injected vehicles. Various diagnostic schemes were implemented through the 70s and into the 80s, all of them proprietary and lacking any sort of standard - often, in those days, diagnosing a problem meant grounding a wire somewhere under your hood, turning the ignition key partway, and counting the number of times your check engine light flashed. Then, if you had paid the exorbitant fee for your car's shop manual, you could look up the blink count and get what was sometimes a generic and not very helpful result.
It wasn't until 1994 that the California Air Resources Board (CARB) announced the requirement that all cars sold in the state from 1996 onward were required to implement a new standardized "On-Board Diagnostics II" (OBD-II) protocol designed in conjunction with the Society of Automotive Engineers. Widespread compliance with OBD-II meant the possibility that a repair shop could buy a single set of tools to run diagnostics on your Honda, your cousin's Ford, and your grandmother's Buick. Over time, with the march of cheaper and faster technology, we've reached the point where the average consumer can hold the same power in the palm of his or her hand.
The most important tool for any car owner
CAN OBDII/EOBD Diagnostic Scanner (EOBD is the equivalent specification to OBD-II for cars sold in the European market) on up into the domain of serious performance tuning devices that can run into the thousands. The model I currently use is the Equus 3100 Innova CanOBD2, which runs around $80, and has some additional features like emissions testing and a USB interface so it can be plugged into a PC to feed diagnostic information to included software for a more detailed analysis of the results. There are only two truly required features, though: the ability to read codes, and the ability to clear them once the underlying problem is fixed.
Reading the trouble codes thrown by your car's computer is easy: just start the car, plug the device into the OBD-II port, and , on most models, hit the "read" button. The device will display one or more codes - some units display only one code on-screen at a time, but will provide up-down buttons or some other mechanism to cycle through the list.
OBD-II trouble codes consist of a letter (P, B, C, or U for powertrain, body, chassis, and undefined, respectively) followed by a four-digit number. The first digit is always 0 if the error code is a generic one defined by the SAE, or 1 if it's a manufacturer-specific code. The next three digits define the subsystem where the error occurred and a specific error number.
Usually the maker of your device will provide a list of trouble codes, if they're not programmed into the scanner itself. If you can't find the list, an online search for "obd-ii codes" will turn up a variety of sites that provide details on the generic and manufacturer-specific codes. Once you've identified the codes your car is sending you, you're armed at the very least with some information about what part of the car is reporting some issue, and depending on the specificity of the codes thrown and your own level of automotive knowledge, you might be able to identify and/or repair the exact issue yourself.
I recommend taking the next step, though, even if you're not normally the type to even look under the hood: head over to Google and do a search on your car's make, model, year, and one or more of the trouble codes you've recorded. For example, "2005 Pontiac Aztec P0741" (yes, there must be somebody out there who bought one of those hideous things!). The information you might find this way includes manufacturer Technical Service Bulletins (TSBs) detailing known issues and fixes related to your trouble code - and you're a lot better off showing up at your dealer to say, "My car is throwing code P1234, which is covered in TSB #5678-9 - has this fix been applied to my vehicle?" than, for instance, "Help! My check engine light is on! Please fix it!"
But beyond the TSBs, you also have a good chance of finding posts on automotive forums by owners of the same model who have experienced the same problem. This is especially true of you own a vehicle with an enthusiast following - a sportscar, truck, or hybrid, for instance - or one that's a very common model such as a Camry. To illustrate how useful this resource can be, I'll provide a recent example from my own efforts to keep the aforementioned beat-up '95 Mustang on the road:
About two years ago, my car started experiencing a rough idle and the check engine light came on. I used my scan tool to pull the code and searched using the method above. The results included a number of helpful posts on some of the bigger Mustang enthusiast forums from people who had experienced the same issue. It turned out that a common problem on these cars is that the throttle position sensor - a small piece of electronics that measures the flow of air coming into the engine - tends to accumulate small dirt particles over time that can eventually lead to inaccurate readings. One post even explained exactly how to fix it, with pictures - and 10 minutes later, with a few turns of a screwdriver and a gentle cleaning with a cotton swab dipped in rubbing alcohol, the rough idle was gone! I cleared the codes with my scanner and drove on, having spent no money on the repair beyond the purchase of the scan tool years before.
Out of curiosity, the next day I called a local dealership's service department and asked them what my trouble code meant. I was told that it's usually a bad throttle body - the larger mechanical assembly the sensor I cleaned is part of - a part costing over $300 and taking 1-2 hours of labor to replace. So in all likelihood my inexpensive code scanner and a few minutes online saved me upwards of $500!