The powertrain control module (PCM) controls the fuel injector and the starter operations based on a vehicle theft deterrent (VTD) password from the vehicle body control module (BCM). When the ignition is first turned ON, the BCM sends a programmed VTD password to the PCM. The PCM acknowledges the password and responds to the BCM that normal fuel injector and starter operation will continue. If the PCM detects an incorrect password due to a VTD system failure or an attempted vehicle theft, DTC 1631 will set. If this DTC sets after the engine is already running, the engine will continue to run and will continue to start on subsequent drive trips as long as the battery is not disconnected. If this DTC sets at PCM power-up, the starter and the engine fueling will be disabled.

A DTC P1631 indicates that the vehicle theft deterrent password that the PCM has learned does not agree with the password being received from the VTD system. This condition can occur if an incorrect key is being used when attempting to start the vehicle or if the BCM has been replaced and the PCM password learn function has not been enabled. If the BCM has been replaced, the password must be relearned. Refer to Diagnostic Starting Point - Theft Deterrent in Theft Deterrent.