GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

Perform the Diagnostic System Check - Door Systems prior to using this diagnostic procedure.

DTC Descriptor

DTC B3127: LF Door Only Unlock Circuit Low

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

BCM B+

1

1

--

--

Driver Door Lock Switch Signal

B3272

1

1

--

Passenger Door Lock Switch Signal

B3272

1

1

--

Driver Door Lock Control

B3127, B3137

B3128, B3138

1

--

Passenger Door Lock Control

B3132, B3137

B3133, B3138

1

--

Rear Door Lock Control

B3132, B3137

B3133, B3138

1

--

Driver Door Lock Switch Ground

--

1

1

--

Passenger Door Lock Switch Ground

--

1

1

--

1. Door Locks Malfunction

Circuit/System Description

The body control module (BCM), upon receipt of a lock switch lock signal, will provide and ground to the door latch unlock control circuit while simultaneously providing voltage to the door latch lock control circuit causing the door latch to perform the lock function. When the BCM receives a lock switch unlock signal, the BCM will provide and ground to the door latch lock control circuit while simultaneously providing voltage to the door latch unlock control circuit causing the door latch to perform the unlock function.

Conditions for Running the DTC

The system voltage must be between 9-16 volts.

Conditions for Setting the DTC

The BCM detects a short to ground in the driver door unlock control circuit while commanding the door latches.

Action Taken When the DTC Sets

The power door locks will malfunction.

Conditions for Clearing the DTC

    • This DTC will change from current to history when the fault is no longer present.
    • A history DTC will clear after 100 consecutive ignition cycles if the condition for the malfunction is no longer present.

Reference Information

Schematic Reference

Door Lock/Indicator Schematics

Connector End View Reference

Power Door Systems Connector End Views

Description and Operation

Power Door Locks 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
    •  Scan Tool Output Controls

Circuit/System Verification

Ignition ON, observe the function of the driver door latch while commanding the Driver Door Unlock and Door Lock with a scan tool. The driver door should perform the LOCK and UNLOCK function.

Circuit/System Testing

  1. Ignition OFF, disconnect the C2 harness connector at the driver door lock.
  2. Connect a test lamp between the control circuit terminal A and ground.
  3. Ignition ON, command the Driver Door Unlock with a scan tool. The test lamp should briefly illuminate during the UNLOCK command.
  4. If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.
  5. If all circuits test normal, test or replace the driver door lock.

Component Testing

  1. Ignition OFF, disconnect the harness connector at the driver door latch.
  2. Install a 20A fused jumper wire between one of the control terminals and 12V. Momentarily install a jumper wire between the other control terminal and ground. Reverse the jumper wires at least two times, the driver door latch should perform the LOCK/UNLOCK functions.
  3. If it does not perform as specified, replace the driver door latch

Repair Instructions

    •  Front Side Door Lock Actuator Replacement
    •  Body Control Module (BCM) Programming/RPO Configuration for BCM replacement, setup, and programming