Why is my check engine light on?

If your check engine light is on, it simply means that the Engine Control Unit (ECU - the computer that controls your car's engine) has found a problem. It could be as serious (and expensive) as a blown head gasket, or as simple as not putting the gas cap on tight enough. Common causes of a check engine light are vacuum leaks, bad oxygen (O2) sensor, sticky EGR valve, or dirty fuel injectors. Some of the problems that make your check engine light come on are harmless, but others could cause serious damage to your vehicle. In other words, if your check engine light comes on it's OK to drive your car (unless it's flashing), but you should get it checked out as soon as possible. You can either take it to a mechanic or read the ODB code(s) yourself. Naturally I recommend reading the code yourself. Even if you decide to take your car to a mechanic to get fixed, knowing the check engine code that made the light come on will help you avoid getting ripped off.

If your check engine light is FLASHING, it means your car's ECU has discovered a serious problem that could result in your car on fire because raw fuel is being dumped into your catalytic converter. If your check engine light is flashing, park the car as soon as you safely can (you obviously don't want to stop in the middle of a freeway) and have it towed to a competent repair shop.

The easiest way to read your car's check engine light code is with an OBD-II scan tool like the Autel MaxiScan MS300 Scan Tool. These cost less than $30 and work with any OBD-II compliant car made since 1996. If your car is 2004 or newer, make sure the scanner you buy supports the CAN protocol or it won't work with your vehicle. Besides reading check engine light codes, an OBD-II scanner will let review the emission readiness status of your car (will it pass smog?) and let you easily turn off (reset) the check engine light.

Maybe - if you live near an AutoZone store, they will read your check engine light code for free and also reset your check engine light. Of course, once you have the code you'll still need to figure out what to do with it...

OK, you've got your check engine light codes, so now what? There are hundreds of generic OBD-II codes so what do they mean? There are two ways to find out: look them up online or use a reference book like the OBD-II & Electronic Engine Management Systems Techbook. Personally, I prefer having the book for the same reason I prefer reading my own codes - it saves me time and it's more convenient. Also, in addition to just a list of OBD-II codes, the Techbook includes a glossary and acronym list to help you understand sometimes cryptic descriptions and a section on component replacement which is extremely helpful if you decide to attempt the repair yourself. Can you save money by trying to find the information on the Internet? Yes, but if you get the book you'll save time so you have to ask how much your time is worth to you. I have good Internet access but I prefer the book - by far.

Of course you can:) The best resource for looking up OBD-II check engine codes is OBD-Codes.com and you can go directly to their ODB-II code list by clicking the links below. The big link goes to their generic OBD-II code list, followed by links to their manufacturer specific code listings.

There are three ways you can reset your check engine light:

OBD-II Scan Tool If you have an OBD-II scan tool (like the Autel MaxiScan MS300 Scan Tool) you can use it to reset your check engine light. This is the method I prefer since I already have the tool and it's most convenient for me to work on my car at my house.

Take it to AutoZone If you take your car to AutoZone to have the Engine Check Light code read, they will also reset it for you. The advantage is it's free. The disadvantage is you have to spend time and gas to drive your car to AutoZone, then wait until they have time to check it for you.

Disconnect the Battery If you don't have an OBD-II code reader and you don't have time to drive to AutoZone, disconnecting your car's battery for about 10 minutes will clear the check engine light. Of course, it will also clear any preset stations you have in your car's radio and possibly other settings, and you won't know what the trouble code was that made the light come on. But hey, at least the light went out - at least for a little while.

Believe it or not, a loose gas cap can indeed make your check engine light come on. Why? Because gasoline vapors are considered to be emissions, and a loose gas cap allows gasoline vapors to leak into the atmosphere. Your car's ECU can detect this and considers it to be a problem with your car's emissions system, so it turns on the check engine light. The good news is that it's an easy problem to fix - just make sure to tighten your gas cap properly each time you fill up (you should turn it far enough to click several times). In most cases the light will clear itself after several fill-ups. If not, or if the light comes back after you clear it, there may be a hole in the filler tube, the tank itself, or one of the connecting gaskets.

If your check engine light code indicates a bad catalytic converter (CAT), you may be in for a shock to your wallet. To replace the CAT on my 2002 Subaru, for example, costs almost $1000. Ouch. However, if the code indicates a bad CAT, don't panic. There are two fairly common causes that are very easy to fix and won't cost a small fortune to fix: the CAT might not be getting hot enough or you may have gotten a tank of bad gasoline. Either of these can cause the ECU to detect a "bad" CAT. An honest shop will tell you this. Other shops will just replace the CAT and pocket the profit.

Not hot enough: To work correctly, your CAT needs to get hot - really hot - to burn off emissions particles. If it doesn't get hot enough, the particles don't burn and can eventually clog up the CAT. If you do only around town driving and short trips, chances are your CAT is rarely getting up to full operating temperature. The fix is easy - periodically take your car on a drive at highway speeds for at least 30 minutes. This will allow your CAT to get hot enough to burn off all the accumulated particles.

Bad (cheap) gasoline: Running a tank or two of bad gasoline (unfortunately not all gas is created equal) through your car can also cause the ECU to detect a "bad" CAT.If your check engine light code indicates a bad CAT and you're using off-brand gasoline or you buy it from a low volume service station, that could be the problem. Clear the light, buy a few tanks of gas from a name brand, high volume station, and take a few nice long drives to get the CAT good and hot. It just might fix the "problem" without having to replace the CAT.