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Check Engine Lights Are Sensitive

Sep 15, 2008
The service engine soon or check engine warning light is your cars friend but it can be very sensitive and annoying. In this quick article I will discuss why the check engine light comes on. I will also touch on a few ways to diagnose codes and turn off the amber check engine light.

The check engine light is turned on directly by the vehicle computer. Most manufacturers call this the PCM. This stands for Powertrain Control Module. You may find other names for this unit but basically it is considered the vehicles main brain.

The PCM watches many different engine sensors. It then divides these sensors into multiple channels and monitors each channel and compares the range of data to the specifications that are in memory. The pcm is monitoring both inputs like coolant temperature and oxygen sensors but also output controls like idle air control motor steps and egr percentage open or closed.

So the amber engine light is turned on when the computer notices a parameter out of range. Keep in mind that output control parameters like the EGR and IAC mentioned above are usually measured with a resistance value or reference voltage to assure proper component operation. And also the check engine light is amber to express caution not red like the oil light that means emergency or turn off the vehicle now or damage your engine.

When the check engine light is turned on the computer is notifying the driver that one of the channels that it is monitoring is out of the specific range. When this light comes on the car may have set a single code or many codes.

This is where diagnosing the check engine light properly will save you money and time in replacing unnecessary parts. Using an automotive scan tool, you can view the data stream for the particular sensors that set the code.

Another good rule of thumb when diagnosing sensor codes is to diagnose the failures that are set with lower numbers, prior to diagnosing the other sensor codes. This is because the higher number codes could be set by the lower number failures. This is not a hard rule and you may have to apply some common sense to this as in a misfire code should be addressed before repairing a rich exhaust code. Since the UN burnt fuel from a cylinder is detected as a rich condition.

To review your computer is monitoring many channels and multiple sensors and simply comparing the data with the parameters that have been programmed into the PCM itself. Above or below these data settings commands the PCM to notify the driver they are polluting the atmosphere by turning on the service engine soon light.

In some cases these out of range readings can be either temporary or intermittent. This is again where the scan tool comes in handy for figuring out why the check engine lamp is illuminated. You can clear the codes and run the vehicle and see if the check engine light comes back on. If it does, then you have a path of diagnosis related to the diagnostic codes that have returned. If the warning light stays off after you clear the code then don't worry about it. This could have been an isolated glitch or an intermittent problem. Also note that some times carmakers change the parameters in the computer memory so a particular code will not be so sensitive. When this happens the manufacturer will issue a technical service bulletin to notify mechanics.
About the Author
Mark Gittelman is an ASE master technician and an auto repair expert. Mark has more than 23 years experience in the auto repair business. His website provides answers to car repair questions. Stop over to see his latest video that shows you the key features of his favorite online car repair manuals.
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