The programmable control module, or in some vehicles, the engine control module or ECM, stores codes when one of the sensors malfunctions or if a sensor sends a wrong signal to the computer. The computer then turns the engine light on, which alerts the driver that something is amiss. You need a scan tool to find out what the codes are, then you must reset the computer with the scan tool.
Plug the scan tool into the data link port. In most vehicles, the data link port is located under the driver side dash, just to the left of the steering column.
Turn the key to the "On" position. Press the "Read" button to get the codes. Write the codes down, then cross-reference them with the code sheet that comes with the scan tool. Repair the problem sensor or the problem that causes the sensor to send a bad signal. The oxygen sensor is a good example of this: It may not be bad, but you might get a code saying it is "bad," when in fact, it is sending wrong information because it received wrong information about the mixture.
Plug the scan tool in again, if you removed it. Turn the key to the "On" position. Press the "Erase" button on the scan tool to reset the computer. Test drive the vehicle. If the engine light comes back on, repeat the process until you have made the all the proper repairs and the engine light no longer comes on.
