For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category

### DTC Descriptor

DTC P1631: Theft Deterrent Fuel Enable Signal Not Correct

### Circuit/System Description

The theft deterrent module (TDM) sends the pre-release password to the engine control module (ECM) via the serial data circuit. The ECM then sends a challenge to the TDM. Both the ECM and TDM perform a calculation on this challenge. If the calculated response from the TDM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

### Conditions for Running the DTC

Ignition is in the ACC or ON position.

### Conditions for Setting the DTC

The ECM receives an incorrect pre-release password from the TDM.

### Action Taken When the DTC Sets

• | The security warning lamp will illuminate.

• | Vehicle starting will disable or delay.

### Conditions for Clearing the DTC

• | A current DTC P1631 will be cleared when a correct pre-release password is received.

• | A history DTC will be cleared after 40 fault-free ignition cycles.

• | The DTC can be cleared using a scan tool.

### Reference Information

#### Schematic Reference

Theft Deterrent System Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Theft Systems Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

Important: Because of normal system communications between the TDM and ECM, any theft deterrent B-codes must be diagnosed prior to the diagnosis of P-codes.

Perform the Programming Theft Deterrent System Components procedure. Refer to Theft Deterrent Control Module Programming and Setup. The ECM should successfully complete the password learn and the vehicle should start.

If the ECM does not complete the learn or vehicle starting remains disabled, replace the ECM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for ECM replacement, setup, and programming