U1255 Serial Data Line Malfunction

Circuit Description

The class 2 serial data line communicates information between the modules. Each module connected to the serial data line is assigned a recognition code, or address. This code is used in order to identify which modules are communicating. The modules communicate with each other by periodically sending state of health (SOH) messages. When the ignition switch is in RUN, each module that is communicating on the serial data line sends an SOH message every 2 seconds. This ensures that the system is operating properly. When the key is turned on, most modules must learn the initialization data from other modules on the serial data line. When a module requests initialization data from another module, and that request is not acknowledged after 5 seconds, the module stores the class 2 initialization error DTC U1255.

Conditions for Setting the DTC

1. The module does not receive the required initialization data on the class 2 serial data line. (The module retries the request for 5 seconds, and then sets DTC U1255.
2. This DTC only sets if DTC U1300 or DTC U1301 are not current.

Action Taken When the DTC Sets

^ DTC U1255 is stored in the module.
^ The module uses the default values for the initialization data that was requested.
^ This DTC does not cause a driver warning message to appear.

Conditions for Clearing the DTC

This DTC clears (the status changes from current to history) under any of the following conditions:

^ At each ignition cycle.
^ A history DTC clears after 50 consecutive ignition cycles if the conditions for setting the DTC are no longer present.
^ Current and history DTCs may be cleared manually by using a scan tool.

Diagnostic Aids

^ DTC U1169 does not normally represent a failure within the module.
^ When loss of class 2 serial data communication DTCs (DTCs with the letter U as the prefix) are stored as history DTCs, know all possible serial data DTCs set in each module before diagnosing the serial data line.
^ If U1169 is stored in the module memory, inspect for the same DTC stored in other modules capable of storing this DTC.
^ If the DTC is a history DTC, the problem may be intermittent.

Test Description

The numbers below refer to the step numbers on the diagnostic table:

2. U1169 is the result of loss of communication with the memory mirror module (MMM). Note that other modules on the class 2 serial data line may also set U1169 if the MMM is not communicating with the serial data line.

4. Clear all DTCs after the repair procedure is complete.