Monitored Components

There are several electrical components that will affect vehicle emissions if they malfunction. If one of these components is malfunctioning, a Diagnostic Trouble Code (DTC) will be set by either the Powertrain Control Module (PCM) or the Engine Control Module (ECM). The Malfunction Indicator Lamp (MIL) will then be illuminated when the engine is running.

These electrically operated components have input (rationality) and output (functionality) checks. A check is done by one or more components to check the operation of another component.

Example: The Intake Manifold Air Temperature (IAT) sensor is used to monitor intake manifold air temperature over a period of time after a cold start. If the temperature has not risen to a certain specification during a specified time, a Diagnostic Trouble Code (DTC) will be set for a problem in the manifold air heater system.

All open/short circuit checks, or any component that has an associated limp-in will set a DTC and trigger the MIL after 1 trip with the malfunction present. Components without an associated limp-in will take two trips to illuminate the MIL.

The PCM and ECM monitor many different circuits in the powertrain system. If the ECM or PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the ECM's or PCM's memory With certain DTC's, if the problem is repaired or ceases to exist, the ECM or PCM cancels the code after 40 warm-up cycles. Certain other DTC's may be cancelled after 1 or 2 good "trips". Refer to Trip Definition. DTC's that affect vehicle emissions illuminate the Malfunction Indicator Lamp (MIL). The MIL is displayed as an engine icon (graphic) on the instrument panel. Refer to Malfunction Indicator Lamp.

Certain DTC's will set a "companion DTC" in the opposite control module. This means that after repair, the DTC must be erased from both modules.

Certain criteria must be met before the ECM or PCM will store a DTC in memory. The criteria may be a specific range of engine RPM, throttle opening, engine temperature or input voltage.

The ECM or PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, assume the DTC criteria requires the ECM to monitor the circuit only when the engine operates between 750 and 2000 RPM. Suppose the sensor's output circuit shorts to ground when engine operates above 2400 RPM (resulting in 0 volt input to the ECM). Because the condition happens at an engine speed above the maximum threshold (2000 rpm), the ECM will not store a DTC.

There are several operating conditions for which the ECM and PCM monitors and sets DTC's. Refer to Monitored Systems, Components, and Non-Monitored Circuits.

Fig. 3 16-Way Data Link Connector:

Technicians must retrieve stored DTC's by connecting the DRB scan tool (or an equivalent scan tool) to the 16-way data link connector. Refer to the Diagnostic Trouble Code chart (list). Remember that DTC's are the results of a system or circuit failure, but do not directly identify the failed component or components.

Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, disconnecting a relay or removing an electrical connector while the engine is running. When a repair is completed and verified, connect the DRB scan tool to the 16-way data link connector to erase all ECM and PCM DTC's and extinguish the MIL.