DTC 1-4: Telephone Module Error

NOTE: Check battery charge and condition.

Delete all DTCs from the OnStar control unit (see HOW TO DELETE THE DTC ).
Turn the ignition switch OFF, and then back to ACC (I).
Enter the self-diagnostic mode and check for the DTCs.
Is DTC 1-4 indicated?
1. YES - Replace the OnStar control unit.
2. NO - Intermittent failure. The system is OK at this time. Check for loose or poor connections.