GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Epica 🢒 Accessories 🢒 Theft Deterrent 🢒 DTC P1626 or P1631

Diagnostic Instructions
    • Perform the Diagnostic System Check - Theft Deterrent prior to using this diagnostic procedure.
    • Review Diagnostic Starting Point - Theft Deterrent for an overview of the diagnostic approach.

DTC Descriptor

DTC P1626: Theft Deterrent Start Enable Signal Lost

DTC P1631: Theft Deterrent Start Enable Signal Not Correct

Circuit/System Description

The engine control module (ECM) is the control center of the fuel injection system. If constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The ECM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the malfunction indicator lamp (MIL), and store a DTC or DTCs which identify the problem areas to aid the technician in making repairs. The ECM detects communication link failure with the immobilizer control unit or an incorrect message identification being received.

Conditions for Setting the DTC

DTC P1626

    • No immobilizer message identification for ECM release time window (1.5 or 2.0 seconds).
    • The ignition switch is turned to ON.
    • The vehicle speed sensor (VSS) is less than 512 km/h (318 mph).

DTC P1631

    • The wrong immobilizer message is received.
    • The ignition switch is turned to ON.
    • The immobilizer option is selected.
    • The ECM release time window expired.
    • The vehicle speed sensor (VSS) is less than 512 km/h (318 mph).

Action Taken When the DTC Sets
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Failure Records buffers.
    •  A history DTC is stored.

Conditions for Clearing the DTC
    • A history DTC will clear after 40 consecutive warm-up cycles without a fault.
    • DTCs can be cleared by using the scan tool.
    • Disconnect the ECM battery feed for more than 10 seconds.

Reference Information

Schematic Reference

Theft Deterrent System Schematics

Connector End View Reference

Body Control System 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

Circuit/System Verification
  1. Ignition ON, observe the DTC information with a scan tool. Verify that DTCs 032, 033, 034, 035, or 036 are not set.
    2. ⇒If any of the above DTCs are set, refer to Diagnostic Trouble Code (DTC) List for further diagnosis.

Circuit/System Testing
  1. Ignition OFF, disconnect the harness connectors for the Body Control Module (BCM) and the Electronic Control Module (ECM). Test the circuit between A7 of the BCM connector and K32 of the ECM connector for an open, short to ground or short to voltage.
    2. ⇒If the circuit tests open, short to ground or short to voltage, repair the circuit.
  2. Clear the DTC's with a scan tool.
  3. Replace the BCM.
  4. Program the ignition key
  5. Ignition ON, observe the DTC information with a scan tool. Verify that DTCs P1626 or P1631 are not set.
    6. ⇒If P1626 or P1631 are set, replace the ECM.

Repair Instructions
    •  Intelligent Switching Unit Replacement
    •  Engine Control Module Replacement