Install a scan tool.

Does the scan tool power up?

Go to Step 2

Go to Scan Tool Does Not Power Up in Data Link Communications

2

1. Turn ON the ignition, with the engine OFF.
2. Attempt to establish communication with the body control module.

Does the scan tool communicate with the BCM?

Go to Step 3

Go to Scan Tool Does Not Communicate with Class 2 Device in Data Link Communications

Important: The engine may start during the following step. Turn OFF the engine as soon as you have observed the Crank power mode.

1. Access the Class 2 Power Mode in the Diagnostic Circuit Check on the scan tool.
2. Rotate the ignition switch through all positions while observing the Class 2 Power Mode on the Scan Tool. Refer to the Serial Data Power Mode table within the Body Control System Description and Operation in Body Control Systems, for a list of the power mode states that correspond to each ignition switch position.

Does the Class 2 Power Mode parameter reading match the ignition switch position for all the selected positions?

Go to Step 4

Go to Power Mode Mismatch in Body Control System

4

Select the BCM display DTCs function on the scan tool.

Does the scan tool display any DTCs?

Go to Step 5

Go to the Symptoms list in the appropriate service category

5

Does the scan tool display B1477 or B1478?

Go to Diagnostic Trouble Code (DTC) List

Go to Symptoms - Retained Accessory Power