Diagnostic System Check - OnStar

A Diagnostic System Check - Onstar (R):

TEST DESCRIPTION
The number(s) 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.
4. The presence of DTC's which begin with "U" indicate some other system control module is not communicating.

For a description of the RPO Code(s) shown in this article/images refer to the RPO Code List found at Vehicle/Application ID RPO Codes