Important Preliminary Checks Before Starting: 

1. Perform A Diagnostic System Check - Lighting Systems before using the symptom tables in order to verify that all of the following are true:

•	There are no DTCs set.

•	The body control module (BCM) can communicate via the serial data link.

2. Review the system operation in order to familiarize yourself with the system functions. Refer to the following:

•	Headlights Circuit Desrciption

•	DRL Circuit Desrciption

•	Fog Lights Circuit Desrciption

•	Exterior Lights Circuit Desrciption

•	Backup Lights Circuit Desrciption

•	Interior Lights Circuit Desrciption

•	Interior Lights Dimming Circuit Desrciption

Several of the symptom procedures ask for a careful visual/physical check. This step is extremely important-it could lead to correcting a problem without further checks and can save valuable time. These checks include the following:

Important: Check for proper installation of electrical components if an intermittent condition exists. Inspect for aftermarket theft deterrent devices, lights and cellular phones. Ensure that no aftermarket equipment is connected to the class 2 circuit. If you can not locate an intermittent condition, a cellular phone signal may cause the condition. Faulty electrical connections or wiring may also be the cause of intermittent conditions. Refer to Testing for Intermittent and Poor Connections in Wiring Systems.

Important: The problem may or may not turn on the SERVICE VEHICLE SOON indicator or store a DTC. Do not use the symptom tables to diagnose intermittent conditions. The malfunction must be present in order to locate the problem.

Poor electrical connections or wiring cause most intermittent conditions. Perform a careful visual/physical check for the following conditions:

Road test the vehicle with a digital multimeter (DMM) J 39200 connected to the suspected circuit. An abnormal voltage when the malfunction occurs is a good indication that there is a malfunction in the circuit being monitored.

Use a Scan Tool in order to help detect intermittent conditions. The scan tool has several features that can be used to locate an intermittent condition. The snapshot feature can capture and store data parameters within the scan tool when the malfunction occurs. This information can then be reviewed in order to see what caused the malfunction.

Refer to a symptom diagnostic procedure from the following list in order to diagnose a symptom that does not set a DTC: