GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2006 🢒 Classic 🢒 DTC 🢒 DTC B3127 or B3128

Circuit Description

The driver door unlock control circuit from the body control module (BCM) controls the driver door unlock relay. When the BCM receives a door unlock signal, the BCM supplies battery positive voltage to the relay.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC B3127 Left Front/Driver Door Only Unlock Circuit Low
    • DTC B3128 Left Front/Driver Door Only Unlock Circuit High

Conditions for Running the DTC

The keyfob unlock button is pressed or a door lock switch is pressed to the unlock position for DTC B3127. The battery positive voltage supplied to the BCM is within normal operating range, 9-16 volts, for DTC B3128.

Conditions for Setting the DTC

The driver door unlock control circuit is shorted to ground for 0.2 second, for DTC B3127. The driver door unlock control circuit is open or shorted to battery positive voltage for 0.2 second, for DTC B3128.

Action Taken When the DTC Sets

The BCM disables the door unlock output for the remainder of the ignition cycle.

Conditions for Clearing the DTC
    • A current DTC B3127 or DTC B3128 clears when the malfunction is no longer present and the ignition switch is cycled.
    • All BCM history codes clear after 100 ignition cycles with no current codes active during the 100 ignition cycles.

Diagnostic Aids

If the DTC is a history DTC, the problem may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Listen for an audible click when the driver door unlock relay operates. Command both the ON and OFF states. Repeat the commands as necessary.

    2.

  2. This step verifies that the body control module is providing voltage to the driver door unlock relay.

    3.

  3. This step tests for an open in the ground circuit to the driver door unlock relay.

    4.

  4. This step tests if voltage is constantly being applied to the driver door unlock relay.

Step

Action

Yes

No

Schematic Reference: Door Lock/Indicator Schematics

Connector End View Reference: Power Door Systems Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the driver door unlock relay ON and OFF.

Does the driver door unlock relay turn ON and OFF with each command?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the driver door unlock relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the driver door unlock control circuit of the driver door unlock relay with a test lamp connected to a good ground.
  5. With a scan tool, command the driver door unlock relay ON and OFF.

Does the test lamp turn ON and OFF with each command?

Go to Step 4

Go to Step 5

4

  1. Connect a test lamp between the driver door unlock control circuit of the driver door unlock relay and the ground circuit of the driver door unlock relay.
  2. With a scan tool, command the driver door unlock relay ON and OFF.

Does the test lamp turn ON and OFF with each command?

Go to Step 8

Go to Step 10

5

Does the test lamp remain illuminated with each command?

Go to Step 7

Go to Step 6

6

Test the driver door unlock control circuit of the driver door unlock relay for a short to ground or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

7

Test the driver door unlock control circuit of the driver door unlock relay for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

8

Inspect for poor connections at the driver door unlock relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 11

9

Inspect for poor connections at the harness connector of the body control module (BCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

10

Repair the open in the ground circuit of the driver door unlock relay. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 13

--

11

Replace the driver door unlock relay.

Did you complete the replacement?

Go to Step 13

--

12

Replace the BCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 13

--

13
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK