EFI & Computerized Engine Troubleshooting
I see this question a lot in car newsgroups: "My car's Check Engine light is on, what does it mean?"
Basically, what it means is that the car's computer, or ECM (Electronic Control Module) has detected a problem with one of its sensors that it uses to gather information, or with one of the devices it controls. Generally, the ECM will store a code in its memory which indicates what particular problem it detected to cause the light to come on. There will be some way to retrieve the code from its memory. This code can be looked up and used to help diagnose the problem.
(If you have a 1996 or newer vehicle and the light is flashing instead of being on solid, see the OBD II section below for info on what this means.)
If you drive around with the Check Engine light on, it may be indicating that your vehicle is putting out more exhaust emissions, using more fuel, or not running as well as it should, so you should get it fixed. In addition, in areas which perform emissions testing of vehicles, your vehicle may receive an automatic test failure if the Check Engine light is on.
Note that just because a trouble code indicates a problem with one particular sensor's input (ex: "oxygen sensor - low voltage") does NOT necessarily mean that replacing that sensor will fix the problem, merely that the indicated problem was the symptom that let the ECM know something is wrong. You need to do some diagnosis to make sure that the sensor itself is the problem. For example, if your fuel filter is clogged up and not allowing enough fuel to be delivered to the engine, you can get trouble codes for oxygen sensor low voltage (too lean) because of it, even though the oxygen sensor itself is fine.
The former Westec Automotive web site had quite a bit of trouble code retrieval and code description info, but it doesn't seem to be up anymore. If you would like to get this info anyway, I've pulled many of the documents on the site off the search engine Google's cache and put them on this site. This Zip file contains HTML files of listings of trouble code meanings for a number of types of vehicles. GM descriptions are in a separate folder in the Zip file called GM. For GM vehicles, you need to know the model year, engine size, and the engine VIN code (the 8th digit of the vehicles's VIN number). You'll have to find the file in that directory that corresponds to your vehicle. This Zip file contains HTML files with descriptions of how to retrieve codes on many vehicles. However, the GM retrieval description included is not of much use because the graphic is missing.
This page also has descriptions of how to retrieve codes from a number of vehicle makes. However, for GM vehicles at least I'd use the files above to interpret the codes rather than their listing, as those files are engine-specific and provide the exact meaning of the code.
Here's a description for retrieving codes from a GM vehicle partially from the Westec Auto page, plus some stuff I added. This will not work on 1996 and later vehicles (which have an OBD II compliant engine management system), or on some 1994 and many 1995 vehicles (which may be completely or partially OBD II). For these vehicles, a scan tool is required to read codes. If you don't have one, you pretty much have to take it to a mechanic. Also, Cadillacs use a different procedure which displays the codes on the climate control panel or driver information center - this procedure can be viewed in the How-To/FAQ section on caddyinfo.com.
By the way, most of the cheaper $30 code readers in stores for GM vehicles appear to be nothing more than a jumper wire, connecting the same pins as the jumper wire/paper clip method, inside a plastic box. You do get a book with the codes listed, but considering that info is free on the web, I think these are quite a ripoff. There are of course the more expensive scan tools that will give you a lot more info than just the codes, as well as allowing you to read codes on vehicles that the flash method can't be used on. These will cost several hundred dollars in most cases.
If you do want the codes in a book, a Chilton or Haynes manual might be a better information, as these contain a great deal of other useful information. I have personally found the Haynes manuals to be the more useful ones. However, since some vehicles have numerous different engines, sometimes with different meanings for the same numerical code, it can get confusing from their listings as to what description corresponds to the code you have retrieved, and I have seen a few instances where the Haynes code listing was simply wrong. Because of this I prefer the online code resources.
OBD II Systems:
1996 and later vehicles will all use an OBD II compliant engine management system. (Some 1994 and 1995 models may also use OBD II.) OBD II is, in part, an attempt to standardize trouble codes and retrieval methods across all makes of vehicles. This page has a listing of what many of the codes mean - namely the generic codes, with a 0 as the first number. The meaning of these codes is the same for all vehicles. The codes with a 1 as the first number are manufacturer specific, this site lists these codes for some makes of vehicles.
This site also has OBD II trouble code descriptions for various vehicles.
OBD II systems also perform a variety of other diagnostic checks not performed with most older systems, such as misfire detection, detection of leaks in the fuel evaporative system, and monitoring of catalytic converter efficiency. Therefore most OBD II vehicles have quite a few more possible trouble codes than pre-OBD II vehicles.
On an OBD II vehicle, when the computer detects a potential problem, it will flag it in its memory, but will generally not turn on the Check Engine light until the second or third trip (ignition cycle) in which the problem is detected. After this, the light will stay on solid until the diagnostic for that trouble code runs and passes on two or three ignition cycles. (For some codes, such as misfire codes, it must additionally run and pass under the same speed and load conditions which were experienced when the code originally set.) Finally, some codes may not turn on the Check Engine light at all - the light only has to come on when the problem indicated by the code could cause the vehicle's emissions to rise more than 50% above federal emissions standards.
One simple thing to check on an OBD II vehicle that has the Check Engine light on is that the gas cap is not loose. If it is, this may be detected as a leak in the fuel evaporative system. As stated above, after you fix the cap, it may take a few trips for the light to go off.
If the Check Engine light is flashing, this means that the computer has detected that your engine is misfiring (i.e. failing to ignite the fuel in the cylinders) badly enough to risk damage to the catalytic converter (misfiring pushes unburned fuel down the exhaust, which can overheat and damage the catalytic converter as it tries to burn it up). If the light is flashing, you should repair your vehicle as soon as possible. Misfiring problems are most commonly caused by such things as a bad spark plug, bad spark plug wire, or ignition coil/module problems.
On most vehicles equipped with OBD II compliant engine management systems (1996 and later), the old manufacturer-specific way of checking codes generally won't work. For these vehicles, you will need a scan tool to read the codes, so unless you want to buy one, basically your only choice is to take the vehicle to a dealer or mechanic. (On Chrysler vehicles, you may still be able to use the old key on-off-on-off-on method to read codes, or so I have heard.) OBD II scan tools are different from other types, older scan tools will not work unless you can get an OBD II cartridge and cable to upgrade it.
If you have any corrections or additional information you would like to share, drop me a line at firstname.lastname@example.org and I will add them to the page.
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