• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

The rain/ambient light sensor module is used to monitor outside lighting conditions. The rain/ambient light sensor module provides a serial data message to the BCM based on outside lighting conditions to determine if outside lighting conditions are correct for either daytime running lamps or automatic lamp control when the headlamp switch is in the AUTO position. In daylight conditions the BCM will command the designated daytime running lamps ON. During low light conditions the BCM will command the low beam headlamps ON. Any function or condition that turns on the headlamps will cancel daytime running lamps operation.

The ambient light sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 V depending on outside lighting conditions. The body control module (BCM) provides a 5 V reference signal to the ambient light sensor and the HVAC control module provides a low reference ground. The BCM monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lamps or automatic lamp control when the headlamp switch is in the AUTO position. In daylight conditions the BCM will command the daytime running lamps ON. During low light conditions the BCM will command the low beam headlamps ON. Any function or condition that turns on the headlamps will cancel daytime running lamps operation.

Daytime Running Lamps will be controlled by different amounts of voltage based on headlight options.

• X88 = Chevrolet
• Z88 = GMC

• Battery voltage must be between 9–16 V.
• Engine ON.
• Headlamp switch in AUTO position.
• Ambient light is day.

The BCM detects a short to ground in the left daytime running lamp control circuit.

The BCM detects a short to ground in the right daytime running lamp control circuit.

The appropriate daytime running lamp is inoperative.

• The condition responsible for setting the DTC no longer exists and the ignition is cycled.
• A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

Headlights/Daytime Running Lights (DRL) Schematics

Master Electrical Component List

Exterior Lighting Systems Description and Operation

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

Control Module References for scan tool information

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the appropriate E13 Headlamp Assembly.
2. Test for less than 5 Ω between the appropriate ground circuit terminal listed below and ground.
• {X88} Terminal 5
• {Z88 with T4A} Terminal 5
• {Z88 with T4F/T4L} Terminal 4
• If 5 Ω or greater
1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 5 Ω
3. Connect a DMM between the appropriate control circuit terminal listed below and ground, ignition ON.
• {X88} Terminal 4
• {Z88 with T4A} Terminal 4
• {Z88 with T4F/T4L} Terminal 7
4. Verify voltage is between 4 V – 12 V when commanding the appropriate Front Park Lamp/Daytime Running Lamp ON with a scan tool.
• If voltage is less than 4 V.
1. Ignition OFF, disconnect the X4 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 Ω in the control circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module
• If voltage is between 4 V – 12 V.
5. Verify voltage is less than 1 V when commanding the appropriate Front Park Lamp/Daytime Running Lamp OFF with a scan tool.
• If voltage is greater than 1 V.
1. Ignition OFF, disconnect the X4 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the control circuit terminal and ground.
• If 1 V or greater, repair the short to voltage on the circuit.
• If less than 1 V, replace the K9 Body Control Module
• If voltage is less than 1 V.
6. Test or replace the E13 Headlamp Assembly.

Perform the Diagnostic Repair Verification after completing the repair.

• Front Headlamp Replacement
• Control Module References for BCM replacement, programming, and setup