U1255

Circuit Description

The Class 2 serial data line is used to communicate information between modules. Each module connected to the serial data line is assigned its own recognition code (address). This code is used to identify which modules are communicating. Modules communicate with each other by periodically sending State of Health (SOH) messages. When the ignition switch is in RUN, each module communicating on the serial data line sends a State of Health (SOH) message every two seconds to ensure that the system is operating properly. At key on, most modules must learn initialization data from other modules on the serial data line (for example, data from panel dimming or personalization). When this module initialization data from another module, and that request is not acknowledged after 5 seconds, this module stores the Class 2 Initialization Error DTC U1255.

Conditions for Setting the DTC

1. The module does not receive required initialization data on the Class 2 serial data line. (The module retried the request for 5 seconds, then sets the DTC.)
2. This DTC only sets if DTC B1328, B1327, B1983, U1300, U1301 is not current in module memory.

Actions Taken When the DTC Sets

- DTCU1255 is stored in module memory.
- The module uses the default values for the initialization data that is requested.
- This DTC does not cause a driver warning message to appear.

Conditions for Clearing the DTC

This DTC clears (status changes from current to history) under any of the following conditions:
- On each ignition cycle.
- A history DTC clears after 50 consecutive ignition cycles if the conditions for setting the DTC are no longer present.
- Use the On-Board clearing DTCs feature.
- Use a scan tool.

Diagnostic Aids

- DTC module U1255 does not normally represent failure within the module.
- When Loss of Class 2 Serial Data Communications 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 U1255 is stored in module, check for the same DTC stored in other modules capable of storing this DTC. The Data Link Connector (DLC) circuit description provides a chart which indicates which modules may set U1255.
- If the DTC is a history DTC, the problem may be intermittant.

Test Description

The numbers below refer to the step numbers on the diagnostic table:
1. Perform the Body Control Module Diagnostic System Check before continuing with the diagnosis of this DTC.
2. U1255 is the result of loss of communication with a module that provides needed initialization data. This step determines this by monitor Class 2.
4. Clear all DTCs after the repair procedure is complete.