The Check Engine Light (which is actually the Malfunction Indicator Lamp or MIL) alerts you when a problem occurs in the engine control system. Depending on the nature of the problem, the Check Engine Light may come on and go off, remain on continuously or flash. Some intermittent problems will make the Check Engine Light come on only while the fault is occurring. When the problem goes away, the Check Engine Light goes off. Other types of problems will turn the Check Engine Light on, and it will remain on until the fault is diagnosed and repaired.
The Check Engine Light can be annoying because it seems to have a mind of its own. The Check Engine Light also tells you nothing about the problem. It might be something serious - or it might not. There is no way to know until you diagnose the vehicle. So you don't know if you should stop immediately or ignore the Check Engine Light and keep on driving.
If no other warning lights are on, and the engine seems to be running normally (no unusual noises, smells, vibrations, etc.), you can assume the fault that is causing the Check Engine Light to come on is probably minor and won't hinder your ability to continue driving. But if other warning lights are on, you should stop and investigate the problem.
When the Check Engine Light comes on, a diagnostic trouble code (DTC) is recorded in the on-board computer memory that corresponds to the fault. Some problems can generate more than one trouble code, and some vehicles may have multiple problems that set multiple trouble codes.
SETTING TROUBLE CODES:
In most older vehicles (those made before 1996), disconnecting the computer's power source or disconnecting a battery cable erases fault codes and turns off the Check Engine Light, at least temporarily. If the problem persists, the code will reset and the Check Engine Light will come back on. But on many newer vehicles, you do NOT want to disconnect the battery because doing so can wipe out the computer's memory settings. This may affect the operation of the transmission, climate control system and other functions.
In 1996 and newer vehicles, a scan tool or code reader must be used to erase codes and turn the Check Engine Light off.
READING FAULT CODES
If your Check Engine light is on, you need to read the code(s) that are causing it to come on with a code reader or scan tool. The tool is plugged into the vehicle diagnostic connector. The 16-pin connector is usually located under the dash near the steering column. When the ignition is turned on (don't start the engine), the tool will display a number that corresponds to a fault code description. The code will tell you which circuit or component is involved, but the code by itself will NOT tell you which part to replace. That requires more diagnosis.
OBD II CHECK ENGINE LIGHT DIAGNOSTICS
Prior to OBD II, fault detection was mostly limited to "gross failures" within individual circuits or sensors. The first generation systems were not capable of detecting misfire, converter problems or fuel vapor leaks. OBD II changed all of that by adding the ability to monitor these things so emission problems can be detected as they develop.
OBD II still uses the Check Engine Light to alert the driver when a fault occurs, and it still stores trouble codes that correspond to specific kinds of problems, but it adds the unique ability to track problems as they develop and to capture a snapshot of what is going on when a problem occurs.
Almost any emission problem that causes hydrocarbon emissions to exceed 1.5 times the federal limit can cause the Check Engine Light to come on with OBD II - even if there is no noticeable drivability problem accompanying the emission problem.
OBD II not only monitors the operation of all the engine's sensors and systems (fuel, ignition, EGR, evaporative emissions, etc.), it also monitors the operation of the catalytic converter and can even detect engine misfires! Anything that could possibly affect emissions is monitored by OBD II, including a loose gas cap!
A misfire will cause the Check Engine Light to flash while the misfire is occurring. A misfire that occurs in a given cylinder will also set a P030X code where "X" will be the number of the cylinder that is misfiring. For example, a P0302 code would tell you cylinder number two is misfiring. But here's the important point: The code does not tell you why the cylinder is misfiring. You have to figure that out by performing other diagnostic tests. The misfire might be due to a fouled spark plug, a bad plug wire, a defective ignition coil in a DIS ignition system, a clogged or dead fuel injector or a loss of compression due to a leaky exhaust valve, leaky head gasket or worn cam lobe.
OBD II monitors the operating efficiency of the catalytic converter with a second oxygen sensor in the tailpipe behind the converter. By comparing upstream and downstream O2 sensor readings, it can determine how well the converter is doing its job. If converter efficiency drops below a certain threshold, OBD II will set a code and turn on the Check Engine Light.
OBD II can detect fuel vapor leaks (evaporative emissions) in the charcoal canister, evap plumbing or fuel tank by pressurizing or pulling a vacuum on the fuel system. If the gas cap is loose or missing, it will detect it, set a code and turn on the Check Engine Light.
In addition, OBD II can also generate codes for various electronic transmission problems and even air condition failures such as a compressor failure.
TWO KINDS OF OBD2 FAULT CODES:
OBD II is capable of generating two types of diagnostic trouble codes: "Generic" codes that are the same for all makes and models of vehicles (these are required by law), and "Enhanced" or "OEM" codes that are unique to specific vehicles. Enhanced codes also cover non-emission related failures that occur outside the engine control system. These include ABS codes, HVAC codes, airbag codes and other body and electrical codes.
The "generic" codes that are common to all vehicle manufacturers can be accessed using any basic code reader or scan tool that is OBD II compliant. Unfortunately, most older scan tools made before 1995 won't work on the newer OBD II systems. You need a scan tool that has the proper hardware and software to read OBD II codes and other diagnostic information. In fact, you must use a scan tool or code reader to read codes on most 1996 and newer vehicles because most newer vehicles do not have manual flash codes. There are some exceptions. Most Nissan models still provide manual flash codes, as do some Dodge models. Most GM, Ford, Honda and Toyota models do not have flash codes, but on some GM vehicles with a driver information display, there may be a procedure for displaying codes manually.
A simple code reader that plugs into the vehicle diagnostic connector can usually be purchased at an auto parts store for under $100. A basic scan tool that can read codes and additional system data (and erase codes) may sell for $100 to $400 depending on its features.
If you do not have a code reader or scan tool, you will have to take your vehicle to a repair facility or auto parts store if you need to diagnose a Check Engine Light problem.
Money Saving Tip: Some parts stores (such as AutoZone) will do a FREE diagnosis for you. They will use a scan tool to tell you what the code is -- but the code by itself doesn't always tell you which part needs to be replaced. Additional diagnostic tests are usually needed to isolate the faulty component.