When a problem is detected with emission control, the Check Engine light comes on and a diagnostic trouble code is stored in the powertrain computer (PCM).

As a result, the code can be read using a scan tool to determine the nature of the emission control problem.

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With OBD II, the Check Engine light will come on anytime emissions exceed federal limits by 50% on two consecutive trips, or there is a failure of a major emissions control system. Furthermore, many areas now check emissions by performing an OBD II plug-in emissions test. The test checks to see that all the OBD II system monitors have run, that the Malfunction Indicator Lamp (MIL) is working (an OFF), and there are NO stored diagnostic trouble codes (DTCs) in the PCM’s memory.

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