For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using the 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 fuel enable password to the engine control module (ECM) via the serial data circuit. The ECM then challenges this password. 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 RUN position.

### Conditions for Setting the DTC

The ECM receives an incorrect fuel enable password from the TDM.

### Action Taken When the DTC Sets

• | The security indicator will illuminate.

• | Vehicle starting will be disabled.

### Conditions for Clearing the DTC

• | A current DTC P1631 will be when a correct fuel enable password is received.

• | A history DTC will be cleared after 40 malfunction-free ignition cycles or when a scan tool is used to clear DTCs.

### Reference Information

#### Schematic Reference

Immobilizer Schematics

#### Connector End View Reference

Immobilizer Connector End Views

#### Description and Operation

Immobilizer Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Data List

• | Scan Tool Data Definitions

### Circuit/System Verification

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 Control Module References

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

### Repair Instructions

Perform the Diagnostic Repair Verification

Control Module References