Conditions for Setting the DTC
WARNING: This page is about a different car, the 2002 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
A check takes place in the PCM as to whether a message is received by the PCM from the EBCM every 100 milliseconds. DTC U1304 can be set anytime the ignition is in the RUN position and the PCM does not receive a message from the EBCM in over 5.6 seconds.
The DTC 1304 is retrieved from the PCM when the PCM is checked for class 2 DTCs. The PCM sets this DTC to inform of a problem on the UART communications circuit.