How does OBDII work?
The Engine Control Module (ECM) or Powertrain Control Module (PCM) continuously monitors the engine for conditions that can cause an increase in tailpipe emissions. If the ECM or PCM detects a potential problem with the engine, a warning light called a "malfunction indicator light" (MIL) on the dashboard is illuminated to alert the driver of a problem. This light is usually identified by the words "Check Engine," "Service Engine Soon," or the word "Check" along with an engine symbol. The ECM or PCM will also store a Diagnostic Trouble Code" (DTC) in the computer's memory. This DTC can be retrieved with a code reader device to help diagnose problems with the vehicle.