The passlock system is designed to prevent vehicle theft by allowing the PCM to disable the fuel injectors unless a correct key code from the passlock sensor via the mechanical key is correct. The passlock system utilizes four components: the lock cylinder, passlock sensor, body control module (BCM) and powertrain control module (PCM). When the engine is cranked, the BCM will look for a password from the passlock sensor. If the password is not received within a certain amount of time or is incorrect, the BCM will send a "disable fuel command" to the PCM over the Class II link. The SECURITY telltale will flash and the engine will not be allowed to start. If the BCM receives the correct password from the passlock sensor that it has stored in memory, it will send it over the Class II link to the PCM. If the PCM passlock code it has stored in memory does not match the BCM password, or if the PCM does not receive a password from the BCM, it will allow the engine to start then stall within four seconds. The SECURITY telltale will flash for 10 minutes if the PCM receives no password, an incorrect password or a disable fuel command. If the password to the PCM is correct, the engine will operate normally. The password is sent continuously by the BCM to the PCM during run mode. If the password becomes lost or incorrect during the drive cycle, the vehicle will be allowed to start on succeeding ignition cycles without the aid of passlock and the SECURITY telltale will be ON steady. This mode is called fail enable mode.

DTC P1631 will set if the PCM receives an incorrect or unrecognized password from the BCM.

DTC P1631 diagnostic runs continuously when engine is running or cranked.

DTC P1631 is a type C DTC.

Diagnose any U-type communication or BCM passlock/BCM voltage DTCs first.

This DTC indicates the PCM received an incorrect password and may have set during the drive cycle. If the PCM detected an incorrect password during the drive cycle, the vehicle will still start.

If the PCM detected an incorrect password from the BCM during an engine crank, the engine will start then stall.

Perform the Security System (Passlock) Relearn Procedure.