U1255

Subject: All GM Vehicles Using Class 2 Communications Diagnostic Strategy for DTC U1000 or U1255 - PIT3154 - (09/10/2004)

Models: (All GM vehicles using Class 2 communications)

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Condition/Concern:
DTC U1000 or U1255 set current or history, with or without other DTCs

Recommendation/Instructions:
DTC U1000 Class 2 Data Link
DTC U1255 Lost Communications

These DTCs set when the control module does not receive a message that it was expecting from another control module, and does not know which control module did not send that message.

1. If the DTC U1000 or U1255 is set in history with other DTCs set current or history, diagnose the other DTCs first.

2. If the DTC U1000 or U1255 set current, this usually indicates a module that is currently not communicating or a configuration issue.

For example, if the control module is configured for an option (ie: Onstar, Keyless Entry, Memory Mirrors, etc.) that the vehicle does not have, it may expect to receive a message regarding this missing feature. This would usually occur due to the control module being recently replaced and incorrectly setup.

3. When the DTC U1000 or U1255 is current, the module that is not communicating may not even be listed on the Tech 2 on the Diagnostic Circuit Check/Class 2 Message Monitor list.

4. If the DTC U1000 or U1255 has set in history without other DTCs, replacing the control module setting the DTC is most likely NOT the solution. The module that has set the code is looking for an input from another module that is not communicating. Since the module did not receive an input that it is expecting to see, it sets a U1000 or U1255 which indicates there was a loss of communications. Look at the customer's complaint (ie: intermittent, erratic tachometer operation); this will probably be a better indication of the control module that is the source of the concern.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition.