Test Description

The numbers below refer to the step number(s) on the diagnostic table.

2: Lack of communication may be due to a partial malfunction or a total malfunction of the class 2 serial data circuit. The specified procedure will determine the condition.
5: The presence of DTCs which begin with "U" indicate some other system control module is not communicating.

Diagnostic System Check - Cellular Communication

Step	Action	Yes	No
1	Install a scan tool. Does the scan tool power up?	Go to Step 2	Go to SCAN TOOL DOES NOT POWER UP
2	Turn ON the ignition, with the engine OFF. Attempt to establish communication with the vehicle interface unit (VIU). Does the scan tool communicate with the VIU?	Go to Step 3	Go to SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE
3	IMPORTANT: The engine may start during the following step. Turn OFF the engine as soon as you have observed the Crank power mode. Access the Class 2 Power Mode in the Diagnostic Circuit Check on the scan tool. Rotate the ignition switch through all positions while observing the ignition switch power mode parameter. Does the ignition switch parameter reading match the ignition switch position for all switch positions?	Go to Step 4	Go to POWER MODE MISMATCH
4	Select the VIU display DTCs function on the scan tool. Does the scan tool display any DTCs?	Go to Step 5	Go to OnStar Symptom Diagnosis
5	Does the scan tool display any DTCs which begin with a "U"?	Go to Step 6	Go to Step 7
6	Does the scan tool display DTC U1500?	Go to Diagnostic Trouble Code (DTC) List	Go to DIAGNOSTIC TROUBLE CODE (DTC) LIST
7	Does the scan tool display DTC B1000, B1004, B1007, or B1009?	Go to DIAGNOSTIC TROUBLE CODE (DTC) LIST	Go to Step 8
8	Does the scan tool display DTC B1327 or B1328?	Go to DIAGNOSTIC TROUBLE CODE DEFINITIONS	Go to Diagnostic Trouble Code (DTC) List