The Powertrain Control Module (PCM) monitors many different circuits in the fuel injection, ignition, emissions and engine systems. If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it will store a Diagnostic Trouble Code (DTC) in the PCM's memory. If the code is applicable to a non-emission related component or system, and the problem is repaired or ceases to exist, the PCM will cancel the code after 40 engine warm-up cycles. A DTC that affects emissions will light up the Malfunction Indicator Lamp (MIL).
Certain guidelines must be met before the PCM will store a code in its memory. The criteria might be a certain range of the engine rpm, engine temperature and/or input voltage to the PCM. The PCM may 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, the DTC criteria may require the PCM to monitor the circuit only when the engine operates between 750-2,000 rpm. If the sensor's output circuit shorted to ground when the engine operated above 2,400 rpm, (with a result of 0 volt input to the PCM), then no DTC would be stored, since the engine condition occurred at an engine speed above the maximum threshold of 2,000 rpm. There are various operating conditions for which the PCM monitors and sets DTC's.
Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For example, disconnecting a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and verified, use Chryslers DRB® or equivalent scan tool to erase all DTC's, thereby shutting off the MIL.
As a functional test, the Malfunction Indicator Lamp (MIL) lights up at the ignition key ON position before engine cranking. Whenever the PCM sets a DTC that affects emissions, it lights up the MIL. If a problem is detected, the PCM sends a message to the instrument cluster, illuminating the lamp. The PCM will light up the MIL only for codes that affect vehicle emissions. The MIL stays on constantly when the PCM has entered Limp-In mode or has found a failed emission component or system. The MIL stays on until the DTC is erased.
The MIL will either flash or light up constantly when the PCM detects active engine misfire. Also, the PCM may reset (turn off) the MIL when one of the following conditions occur: