The Check Engine Light (MIL)

The check engine light, often referred to as the Malfunction Indicator Lamp or MIL, allows the PCM to alert the driver that an emissions related problem has occurred or is currently occurring. The check engine light is located on the dashboard panel and is colored red or yellow. The light can be labeled Check Engine or Service Engine Soon or may be a symbol of an engine with the word Check in the middle.

The PCM controls the operation of the check engine light and conducts a bulb check whenever the engine is started, informing the driver that the light is functioning properly. Under normal operating conditions the check engine light illuminates when you insert the key and turn the vehicle ignition toward the position that starts the vehicle engine. A few seconds after the engine starts the check engine light is turned off by the PCM. If the PCM continues to command the check engine light to remain illuminated, an emissions related problem is indicated. If the check engine light does not illuminate during the bulb check, a burned out check engine light bulb or system tampering is indicated.

A vehicle will fail the (NYVIP2) inspection if the bulb check of the check engine light does not operate correctly during testing. With OBD II, the check engine light is illuminated only for emissions related failures, separate warning lights are used for other non-emissions problems such as low oil pressure, overheating and charging system problems. Other vehicle systems such as air bags, antilock brakes and tire pressure monitoring will also have their own dashboard panel indicator lights. Some vehicles have maintenance reminder lamps, or service interval lamps that should not be confused with the check engine light and MIL.

Whenever the PCM commands the check engine light to illuminate a diagnostic trouble code (DTC) is also recorded in the computer's memory that corresponds to the fault. Some problems can generate more than one fault code, and some vehicles may suffer from multiple problems that also set multiple codes. With some types of intermittent problems the PCM will command the light to illuminate only while the fault is occurring. When the problem goes away the light is commanded off. Other types of problems will turn the light on, and it will remain on until the fault is diagnosed and repaired. The PCM can turn the light off after several warm-up cycles if the problem does not recur. For example, if the light is on because of a misfire or a fuel delivery problem, and the problem does not recur after three drive cycles (under the same driving conditions), the check engine light will be commanded off. 

This self correcting feature can be very beneficial when it comes to correctly repairing an identified fault. For instance, the PCM has detected an oxygen sensor circuit problem and you replaced the oxygen sensor as the repair. If replacing the sensor actually fixed the vehicle the PCM will turn the check engine light off in a few warm-up cycles. If the vehicle was due for an emissions inspection and you had cleared the codes with an OBD II scan tool, all the monitors would have to execute prior to having the vehicle inspected.

When the PCM commands the check engine light to be illuminated continuously, this means that a fault condition has been detected that could possibly be causing the vehicle to exceed federal limits by 1.5 times. With the steady light there is no need to panic, the malfunction does not require immediate attention, but should be addressed in the near future before more serious problems evolve. If the PCM detects the possibility of a serious malfunction, the light is commanded to flash on and off while the engine operates. This malfunction requires immediate attention and you should only operate your vehicle if absolutely necessary. System faults that cause the check engine light to flash could result in serious damage to the catalytic converter or engine.

There are consequences to manually turning off the check engine light, especially if your vehicle is due for an OBD II emissions test. Erasing stored codes with an OBD II scan tool also resets all of the monitors back to zero, so allow plenty of time for the monitors to run before driving back to the emissions test facility. If any of the erased fault codes reappear, it may prevent one or more of the monitors from completing. 

The old school method of disconnecting the battery erases the memory in the PCM, including stored fault codes and previous OBD monitor test results. It's like resetting the clock back to zero. Consequently, it may take several days (or even weeks) of driving before all of the monitors will run, allowing your vehicle to be tested.