Check Engine Light 101
This has probably happened to you at least once. You're driving your car and it is running fine. All of a sudden a red or yellow warning light flashes on "Check Engine". Of course this usually occurs when you need the car most.
What do you do? What does it mean? Is it safe to drive? Is the car going to break down immediately? Is more damage going to occur the more I drive it? Is it going to blow up? It can be an anxious and stressful time when you don't understand what is happening with your car and don't know what to do about it.
Depending on the age of your car, the pesky light may read "Check Engine, " "Power Loss," "Service Engine Now," or "Service Engine Soon." The lights are connected to computers, which monitor engine and transmission sensors to determine if there is a failure in the system that may cause an increase in harmful emissions. The computer system is called On Board diagnostics (OBD).
On Board Diagnostics continuously monitors sensors and actuators. It actively tests some systems for proper operation while the car is being driven, continuously checking for fuel control and engine misfire. Also, once per trip the OBD checks catalyst efficiency, exhaust gas recirculation operation, evaporative system integrity, oxygen sensor response, and oxygen sensor heaters. The "Check Engine Light" or "Malfunction Indicator Light (MIL)" will illuminate if there is a failure in the emissions system. The light will also blink on and off if there is engine misfire severe enough to damage the catalytic converter.
As you can see, many of your car's systems have sensors and actuators that, if malfunctioning, effect emissions and can cause one of these warning lights to come on. That is why, although you may have had a recent repair to turn off the light, other sensors or actuators can malfunction causing the light to come on again shortly. This can become frustrating for the car operator who does not understand what these lights mean or which systems are malfunctioning.
One of the common questions asked is "will my car pass the state emissions test if the check engine light is on?" Unfortunately the car will not pass, and when the light is on you will not even be able to have the emissions tested until the problem is corrected, the computer reset, and the light shut off.
How is the problem diagnosed and repaired? Trouble-shooting requires checking OBD to see if the computer has set a trouble code. This is done either by putting the computer through a self-diagnostic test or by plugging a special scan tool into the diagnostic connector to access the computer.
A trouble code does not necessarily mean a sensor or actuator is bad. It only means a problem has been detected in that particular circuit. It could be the sensor, the wiring, or a connector somewhere in the wiring. To isolate the fault, a sequence of further diagnostic tests usually has to be performed. Intermittent faults are hardest to find, and some sensor problems may not generate a trouble code at all.
It must be added that repairing a problem, because it is a step-by-step procedure, requires that the first malfunction(s) found must be fixed before the system can be tested again. It is not uncommon to find other faults after the first malfunction is repaired, which requires further diagnosis and repair.
In conclusion, although your car may not be about to "blow up," no warning light should be ignored. The problem can be as simple as a loose gas cap or as serious as failure of a component critical to the engine management of your car. An ignored warning light may cost you much more in repairs or leave you stranded on the side of the road. It is always best to have it checked as soon as it occurs.