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Tech Forum: Engine Management


QUESTION: Why does the “Check Engine” Light Come On?

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ANSWER: That’s a question that many people ask. Most people think the Check Engine Light comes on if an engine has any kind of problem whatsoever. But guess what? They’re wrong!
The Check Engine Light only comes on if a fault is emissions-related. The fault does not actually have to cause a measurable increase in emissions, but if the computer thinks the fault might cause emissions to increase, it will log a diagnostic trouble code (DTC) and turn on the light.

The Check Engine Light, which is officially called the Malfunction Indicator Lamp (MIL), is essentially an emissions warning light and nothing more. It does not come on when the oil needs to be changed, if the oil level is low or if the engine is overheating. There are other warning lights for these kind of problems.


Many problems that trigger the light are relatively minor and have little or no effect on engine performance. But because these problems might cause an increase in emissions, the onboard diagnostic (OBD II) system is required by law to monitor the fault, log a code and turn on the MIL anyway. The reason for doing this is because a fault has the potential for increasing emissions beyond federal limits. On late-model cars, the tailpipe and evaporative emission standards are very strict. Consequently, it doesn’t take much to push emissions over the limit.

A loose gas cap can trigger the light. Why? Because fuel systems on late-model vehicles are sealed to prevent the escape of fuel vapors into the atmosphere. If the gas cap isn’t tightened all the way, or the cap leaks because of a damaged seal, OBD II will catch the fault, set a code and turn on the MIL.


Misfires can cause a huge increase in hydrocarbon emissions as well a loss of performance, rough idle and other problems such as converter damage if the misfire is steady rather than intermittent. Misfires can be caused by a lot of things including worn or fouled spark plugs, bad plug wires or ignition coils, dirty fuel injectors, air leaks or other conditions that cause the air/fuel mixture to run leaner than normal, or even a loss of compression in one or more cylinders because of a burned valve or blown head gasket. All engines will experience an occasional misfire, but if the misfire rate exceeds a certain threshold, OBD II sees it as a problem, sets a code and turns on the light. Sometimes, the MIL will flash while the vehicle is being driven. This is a warning to the driver that the engine is misfiring under load.


Sensor failures or electronic faults within the powertrain control module (PCM) itself can also turn on the MIL. If a sensor is not generating a good signal, or the signal does not make sense for the operating conditions at the time (a wide open signal from the throttle position sensor while the engine is idling, for example), it is recognized as a fault.

In some cases, loss of a key sensor signal or bogus information from the sensor will wreak havoc on the operating logic of the engine management system. On some systems, the PCM can substitute a “simulated” value for a missing sensor signal, but it depends on the type of fault and the sensor that is involved.


A coolant sensor that always reads cold, for example, will prevent the engine management system from going into a “closed loop” mode of operation. This means the PCM ignores the signal from the oxygen sensor and keeps the fuel mixture rich because that’s what a cold engine likes. If the engine is warm, though, staying in open loop too long increases emissions and fuel consumption.

A dirty mass airflow sensor, on the other hand, may mislead the PCM into thinking the engine is using less air than it actually is. This can cause a lean fuel condition, but the PCM may be able to compensate by using other sensor inputs to check the accuracy of the mass airflow sensor’s input.
One very important point to keep in mind with respect to the Check Engine Light on OBD II vehicles is that you must use a code reader, scan tool or scanner software to read and clear codes. There are no manual flash codes on OBD II vehicles. Disconnecting the battery in an attempt to erase a code so the MIL will go off is also a very bad idea because it can cause the PCM to forget learned settings. This may cause additional driveability problems, and may even require using a scan tool to “reset” lost information.

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