What Does the Check Engine Light Mean ?

Most cars have a computer installed in the engine.  The computer or the Electronic Control Module (ECM) controls the car’s operation.  It keeps the car running smoothly and efficiently, and it controls the automatic transmission.  There are many sensors in the engine and transmission of a car.  When the computer senses a problem, the “CHECK ENGINE” or “SERVICE ENGINE” light will come on the dash to alert the driver that the car needs maintenance.

The only way to know exactly what the computer is sensing is to take the vehicle to a mechanic who will connect the car’s ECM to a scanner that can read the trouble code.  Once the code is retrieved, the mechanic can look up the code in the manufacturer’s service manual for that model and tell you about the problem.  The code will not tell the mechanic exactly what is wrong with the car, but he or she can then perform a series of test to isolate the issue.  The manufacturer’s service manual also lists recommended tests for components under each code. 

If the “check engine” light comes on when you start the car, but does not go off within a few seconds, this is an indication that the car’s ECM computer has detected a problem.  A flashing “check engine” light  is an indication that the ECM has detected that the engine is misfiring.  An engine that is not firing properly, or “missing,” can cause damage to parts like the catalytic converter.  A catalytic converter is an expensive repair, sometimes topping $1000 or more.  Often a dealer will tell you that the “check engine” light does not mean anything and to just ignore it. Sometimes a dealer will tell you that the “sensor” is dirty and it is reading a problem that does not exist.  You should ask the dealer to have the car scanned by a mechanic for problems.  If the dealer hesitates or refuses to scan the ECM or to perform the appropriate and recommended tests to resolve the issue, do not buy that used car.

You can drive a vehicle for a while after the “check engine” light comes on, but you risk damaging the component that the ECM sensed as problematic.  Occasionally a dealer will tell you that disconnecting the battery for a few minutes will reset the sensor.  This is true for some cars, but not all ECMs will reset when the battery is disconnected.  Resetting the ECM does fix the problem that caused the “check engine” light to come on in the first place.  It just hides the problem until the ECM detects it again and sends the signal to the driver.  Disconnecting the battery may also reset the code in the ECM called a “readiness code.”  This code indicates that emission control devices, like the catalytic converter, have undergone self diagnostics by the ECM.  An erased “readiness code” will cause the car to fail an emissions test, which can cost you more money. 

The “check engine” light and the ECM are designed to alert the driver to potential or actual problems with the engine, the transmission and other critical components.  Ignoring this warning can result in expensive repairs.  Be wary of the car if the “check engine” light is on.