U1128

DTC MMM U1128-Loss Of Serial Communications (IRC):

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.

CONDITION 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 does not receive an SOH message from the IRC after �4mdu~�0m. consecutive monitoring cycles.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The MMM detects an SOH from the IRC.
^ 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.
^ Clear all DTCs and cycle ignition to see if DTC was set erroneously.
^ If there are multiple DTCs, check for an intermittent short to ground in CKT 1807 (PPL).
^ If the DTC is current, attempt to establish communications with the IRC using the scan tool.
- If communications cannot be established with the IRC, refer to Audio Systems. Accessories and Optional Equipment
- If communications can be established with the IRC, then replace the MMM.
^ If the DTC is history, check for intermittent open in CKT 1807 (PPL).