DTC U1255 - Serial Data Line Class 2 Initialization Error: Description
WARNING: This page is about a different car, the 1997 Cadillac Seville, 1997 Cadillac Eldorado, and 1997 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.
While the ignition switch is in RUN position, the SDM sends messages to other modules communicating on Serial Data Line Class 2.