The ambient light sensor is a light sensitive transistor that varies the voltage signal to the body control module (BCM) based on outside lighting conditions. When the ambient light sensor indicates daytime conditions with the transmission in gear, the body control module (BCM) grounds the DRL relay control circuit, energizing the DRL relay. With the DRL relay energized, voltage is applied through the relay switch contacts, the DRL 1 fuse, and the DRL 2 fuse to the left and right low beam headlamps. Any function or condition that turns the headlamps ON will cancel DRL operation.

Schematic Reference

Headlights/Daytime Running Lights (DRL) Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Turn the headlamp switch to the headlamps ON position, the low beam headlamps should illuminate.

⇒	If the low beam headlamps do not illuminate, refer to Headlamps Malfunction.

2. Headlamps OFF, command the Low Beams ON and OFF with a scan tool. The headlamps should turn ON and OFF between the commanded states.

⇒	If the low beam headlamps are always ON, refer to the Headlamps Malfunction.

⇒	If the low beam headlamps are always OFF, refer to the Headlamps Malfunction.

3. Headlamps OFF, command the DRL ON and OFF with a scan tool. The headlamps should turn ON and OFF between the commanded states.

⇒	If the low beam headlamps are always ON, refer to Daytime Running Lamp Malfunction.

⇒	If the low beam headlamps are always OFF, refer to the Daytime Running Lamps Malfunction.

4. Ignition ON, carefully supply sufficient light to simulate daylight conditions to the ambient light sensor for more than 30 seconds. Observe the scan tool Ambient Light Status parameter. The reading should be Day.

⇒	If not the specified value, refer to the Ambient Light Sensor Malfunction.

Ambient Light Sensor Malfunction

1. Ignition OFF, disconnect the harness connector at the ambient light sensor.
2. Ignition OFF, test for less than 5 ohms between the ground circuit terminal B and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Ambient Light Sensor parameter is greater than 4.65 volts.

⇒	If less than the specified range, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the BCM

4. Install a 3A fused jumper wire between the signal circuit terminal A and ground. Verify the scan tool Ambient Light Sensor parameter is less than 1.96 volts.

⇒	If greater than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, replace the ambient light sensor.

Daytime Running Lamps Malfunction

1. Ignition OFF, disconnect the X3 harness connector at the underhood fuse block.
2. Connect a test lamp between the relay control circuit terminal 40 and B+.
3. Ignition ON, command the DRL lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF.

⇒	If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. Connect the X3 harness connector at the underhood fuse block.
5. Ignition OFF, disconnect the inoperative low beam headlamp.
6. Test for less than 5 ohms between the listed ground circuit terminal below and ground.

•	Headlamp terminal A - w/o V22

•	Low Beam Headlamp terminal B - with V22

⇒	If greater than the specified range, test the ground circuit for an open/high resistance,

7. Connect a test lamp between the listed control circuit terminal below and ground.

•	Headlamp terminal C - w/o V22

•	Low Beam Headlamp terminal A - with V22

8. Command the DRL lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF.

⇒	If the test lamp is always ON, test the control circuit for short to voltage. If the circuit tests normal, replace the underhood fuse block.

⇒	If the test lamp is always OFF, test the control circuit for short to ground or an open/high resistance. If the circuit tests normal, replace the underhood fuse block.

9. If all circuits test normal, replace the inoperative low beam headlamp.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.