[jmsspratlin]

Originally Posted by Easy Ed

Found this here

http://myweb.accessus.net/~090/dtcobd2n.html#typ

Hope it helps explain.

Easy Ed


This information applies to all GM vehicles Using Class 2 communication.
DTC U1000 or U1255 may set current or history, with or without other DTCs.
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 the 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 is 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 (i.e., 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 set up.
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 that set 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. Because 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 concern (i.e., intermittent, erratic tachometer operation). This will probably be a better indication of the control module that is the source of the concern.

Ed seems to have all the answers.