Engine Control System & Engine Component Tests: Operation: System

The Powertrain Control Module (PCM) monitors many different circuits in fuel injection, ignition, emission and engine systems. If PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in PCM's memory. If code applies to a non-emissions related component or system, and problem is repaired or ceases to exist, PCM cancels code after 40 warm-up cycles. Diagnostic trouble codes that affect vehicle emissions illuminate Malfunction Indicator Lamp (MIL).

Certain criteria must be met before PCM stores a DTC in memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to PCM. The PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of DTC criteria for circuit has not been met. For example, assume diagnostic trouble code criteria requires PCM to monitor circuit only when engine operates between 750 and 2000 RPM. Suppose sensor's output circuit shorts to ground when engine operates above 2400 RPM (resulting in 0 volt input to PCM). Because condition happens at an engine speed above maximum threshold (2000 RPM), PCM will not store a DTC. There are several operating conditions for which PCM monitors and sets DTC's.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark test may set misfire code. When a repair is completed and verified, use DRBIII(R) scan tool to erase all DTC's and extinguish MIL.