P1631

Circuit Description
The vehicle theft deterrent system, is designed to prevent vehicle theft by disabling the engine unless the lock cylinder is correctly engaged by a mechanical key. The theft deterrent system utilizes four components for theft detection, the lock cylinder, ignition switch, the Body Control Module (BCM) and the Powertrain Control Module (PCM).
When starting the engine, the PCM looks for a password from the Body Control Module (BCM) through the Class II serial data circuit. If the password is not recognized or not present, the PCM will disable the engine. There are two modes of tamper detection:
^ No Password Received - Engine will start and stall quickly. THEFT SYSTEM telltale will flash on the IPC.
^ Incorrect or Disable Password Received - (More than three invalid passwords are received) - The engine is disabled for at least 10 minutes and the THEFT SYSTEM telltale will illuminate solid on the Instrument Panel Cluster (IPC) for the 10 minutes.
After the vehicle has passed theft detection, the PCM will continue normal engine operation.

Conditions for Setting the DTC
An unrecognized or no password is sent to the PCM from the BCM.

Action Taken When the DTC Sets
^ The PCM will not illuminate the Malfunction Indicator Lamp (MIL).
^ The PCM will store conditions which were present when the DTC set as Fail Records data only. This information will not be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC
^ A history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ DTC can be cleared by using a scan tool.

Diagnostic Aids
Attempting to start the vehicle by bypassing the theft deterrent system or substituting parts without performing the password learn procedure may set a DTC P1631.
If no password is received a DTC P1610, P1626 and P1630 may be set.
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.
Any circuitry, that is suspected as causing an intermittent complaint, should be thoroughly checked for the following items:
^ Backed out terminals or improper mating.
^ Broken locks.
^ Improperly formed or damaged terminals.
^ Poor terminal to wiring connections.
^ Physical damage to the wiring harness.
^ Corrosion.
If the serial data loss occurred after a good theft password has been received, the PCM must be reprogrammed and the password learn and the crankshaft position variation learn procedures must be completed to allow the theft system to operate properly.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table:
1. The Diagnostic System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. The scan tool can display whether or not the BCM has received and learned the password from the lock cylinder. If the password is not received, the reason the PCM set this DTC may due to a password that has not yet been learned due to a component being replaced, or a problem in the BCM or a VTD component.
3. The theft deterrent password resides in the Body Control Module (BCM). Anytime a VTD component is replaced, the Powertrain Control Module (PCM) must learn the new BCM password.
4. A BCM Passlock theft deterrent DTC being set indicates that a theft deterrent component may be malfunctioning.
5. If no vehicle theft deterrent DTCs were set and the BCM has not learned (received) a password from the ignition lock cylinder, the BCM may need to learn the password or be malfunctioning.
6. Replacement PCMs must be programmed. A reprogrammed PCM does not require the password learn procedure to be performed. The password will be learned on the first ignition cycle. The replacement PCM must be programmed and the crankshaft position system variation procedure must be performed.
5. If no malfunctions have been found at this point and no additional DTCs were set, refer to Diagnostic Aids above for additional checks and information.