U1255 MMM Generic Loss of Serial Communications

DTC MMM U1255-Generic Loss Of Serial Communications:

CIRCUIT DESCRIPTION
While the ignition switch is in RUN, modules communicating on the Class 2 data bus send a "State of Health" (SOH) message every 5 seconds to ensure that the system is operating properly.

CONDITIONS FOR SETTING THE DTC
^ This DTC will set only if DTC MMM B1982, DTC MMM B1983, or DTC MMM U1300 is not current.
^ The MMM requests a message for initialization and the following set of criteria is met:
- The request is not acknowledged after all message retries are sent out.
- The requested information has an internal fail soft value.
- The source ID with the desired information is not known.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The conditions for setting the DTC are no longer met.
^ The DTC will clear on every Power Mode Initialization.

DIAGNOSTIC AIDS
^ This serial data DTC does not normally represent a failure within the MMM. This is not true when there is an intermittent connection of the Class 2 data line(s) to the MMM. When serial data codes are history, know all possible serial data DTCs and which ones are set in each module, before diagnosing the Class 2 concern.
^ If there are multiple DTCs, check for an intermittent short to ground in CKT 1807 (PPL).
^ If the DTC is history, check for intermittent open in CKT 1807 (PPL).
^ If DTC MMM U1255 is set, clear DTCs and cycle ignition. If DTC MMM U1255 is still current, ensure that all modules are functioning properly and the Class 2 data bus is not shorted to ground or voltage.
^ If the Class 2 data bus and all the modules on it are working properly, check the MMM Class 2 terminal connections. If OK, replace the MMM.