The numbers below refer to the step numbers on the diagnostic table.
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.
The presence of DTCs which begin with "U" indicate some other system control module is not communicating.
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 in Data Link Communications  | 
 Does the scan tool communicate with the VIU?  | Go to Step 3  | Go to Scan Tool Does Not Communicate with Class 2 Device in Data Link Communications  | |
3  | Select the VIU display DTCs function on the scan tool. Does the scan tool display any DTCs?  | Go to Step 4  | Go to OnStar Symptom Diagnosis  | 
Does the scan tool display any DTCs which begin with a "U"?  | Go to Step 5  | Go to Step 6  | |
5  | Does the scan tool display DTC U1500?  | Go to Diagnostic Trouble Code (DTC) List in Data Link Communications  | |
6  | Does the scan tool display DTC B1000, B1004, B1007, or B1009?  | Go to Diagnostic Trouble Code (DTC) List in Body Control System  | Go to Step 7  | 
7  | Does the scan tool display DTC B1327 or B1328?  | Go to Diagnostic Trouble Code (DTC) List in Engine Electrical  |