The OBD-II connector is primarily used by mechanics to quickly analyze and troubleshoot problems with a vehicle. (See “The OBD-II Connector” on page 17 for help locating the OBD connector.) When a vehicle experiences a fault, it saves information related to that fault and triggers the engine warning light, also known as the malfunction indicator lamp (MIL). These routine diagnostic checks are handled by the vehicle’s primary ECU, the powertrain control module (PCM), which can be made up of several ECUs (but to keep the discussion simple, we’ll refer to it only as the PCM).

If you trigger faults while experimenting with ...

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