GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2005 🢒 Century 🢒 DTC 🢒 DTC B2578

Circuit Description

The body control module (BCM) controls the automatic headlamp relay through the headlamp relay control circuit. The BCM grounds the headlamp relay control circuit, energizing the automatic headlamp relay.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2578 Headlamp Control Circuit High

Conditions for Running the DTC

The BCM must be commanding the headlamp relay to energize.

Conditions for Setting the DTC
    • The BCM detects a short to battery positive voltage in the headlamp relay control circuit when the BCM tries to energize the automatic headlamp relay.
    • The condition above must be present for more than 0.3 seconds.

Action Taken When the DTC Sets

The BCM stores DTC B2578 in memory.

Conditions for Clearing the DTC
    • A current DTC B2578 will clear if the BCM no longer detects a short to battery positive voltage in the headlamp relay control circuit, after the ignition is cycled.
    • A history DTC B2578 will clear after 100 consecutive ignition cycles if the condition for the malfunction is no longer present.
    • The BCM receives a clear code command from the scan tool.

Diagnostic Aids
    • If the DTC B2578 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • The following conditions may cause an intermittent malfunction to occur:
       - An intermittent short to battery in the automatic headlamp relay coil control circuit.
       - The automatic headlamp relay coil is intermittently shorted to battery.
       - The BCM is intermittently shorted to battery.

Step

Action

Yes

No

Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics

Connector End View Reference: Master Electrical Component List in Wiring Systems

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 automatic headlamp relay ON and OFF.

Does the automatic headlamp relay turn ON and OFF with each command?

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the automatic headlamp relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Connect a test lamp between the control circuit of the automatic headlamp relay and the coil side supply circuit of the automatic headlamp relay.
  5. With a scan tool, command the headlamp relay ON and OFF .

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

Go to Step 5

Go to Step 4

4

Test the control circuit of the automatic headlamp relay for a short to battery positive voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 7

Go to Step 6

5

Replace the automatic headlamp relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 7

--

6

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

Did you complete the replacement?

Go to Step 7

--

7
  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