Diagnostic Aids

WARNING: This page is about a different car, the 2004 Pontiac Aztek and 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

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.
An intermittent short to voltage in the following locations may cause an intermittent malfunction to occur:
- The headlamp relay control circuit
- The headlamp relay
- The BCM

DTC B2578

Step	Action	Yes	No
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics Connector End View Reference: Master Electrical Component List
1	Did you perform the Diagnostic System Check?	Go to Step 2	Go to Diagnostic System Check - Lighting Systems
2	Install a scan tool. Turn ON the ignition, with the engine OFF. With a scan tool, command the HDLP low beam relay ON and OFF. Does the HDLP low beam relay turn ON and OFF with each command?	Go to Diagnostic Aids	Go to Step 3
3	Turn OFF the ignition. Disconnect the HDLP low beam relay. Turn ON the ignition, with the engine OFF. Probe the coil side supply circuit of the HDLP low beam relay with a test lamp that is connected to a good ground. Does the test lamp illuminate?	Go to Step 4	Go to Step 10
4	Connect a test lamp between the coil side supply circuit of the HDLP low beam relay and the low beam headlamp relay control circuit. With a scan tool, command the HDLP low-beam relay ON and OFF. Does the test lamp turn ON and OFF with each command?	Go to Step 8	Go to Step 5
5	Does the test lamp remain illuminated with each command?	Go to Step 7	Go to Step 6
6	Test the control circuit of the HDLP low beam relay and the headlamp relay control circuit 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 13	Go to Step 9
7	Test the control circuit of the HDLP low beam relay and the headlamp relay control circuit for a short to ground. 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 a poor connection at the HDLP low beam 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 a high resistance 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	Replace the underhood fuses block. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems. Did you complete the repair?	Go to Step 13	-
11	Replace the HDLP low beam relay. Refer to Relay Replacement (Attached to Wire Harness) or Relay Replacement (Within an Electrical Center) in Wiring Systems. Did you complete the replacement?	Go to Step 13	-
12	IMPORTANT: Perform the set-up procedure for the Body Control Module. Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control Systems. Replace the body control module. Refer to Body Control Module Replacement in Body Control System.Did you complete the replacement?	Go to Step 13	-
13	Use the scan tool in order to clear the DTCs. 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