U1255 Serial Data (Class II) Line Fault - HVAC

DTC U1255 Serial Data (Class II) Line Fault:

CIRCUIT DESCRIPTION
The heater-A/C control continuously communicates with other modules using the class II serial data link. The other modules connected to the class II serial data link are the following:
^ Body Control Module (BCM)
^ Data Link Connector (DLC)
^ Electronic Brake Control Module (EBCM)
^ Instrument Cluster
^ Powertrain Control Module (PCM)

In addition, a scan tool can he connected to the DLC connector to access the class II serial data link for diagnostic purposes.

CONDITIONS FOR SETTING THE DTC
^ The heater-A/C control will perform an internal check on communication with other components on the class II serial data link.
^ The heater-A/C control detects a fault the class II serial data link which may or may not he caused by an external fault.

ACTION TAKEN WHEN THE DTC SETS
Stores a DTC U1255 in the heater-A/C control memory.

CONDITIONS FOR CLEARING THE DTC
^ Under all fault conditions, the ignition must be cycled before this DTC can change from current to history.
^ A history DTC will clear after 100 drive cycles if the condition for the malfunction is no longer present.
^ Using a scan tool.

DIAGNOSTIC AIDS
^ DTC U1255 indicates that the Class II serial data link is open or has been shorted to (B+/ground). The heater-A/C control sends DTC information to the scan tool via the Class II serial data link. If the Class II serial data link is currently open or shorted to (B+/ground), the scan tool will be unable to communicate with the heater-A/C control. Therefore, you will be unable to obtain a DTC U1255 as a current DTC. However, if the scan tool is able to communicate with the heater-A/C control and DTC U1258 is read, it will be a history DTC. Wiggle wires and connectors when performing circuit checks on the Class II serial data link. This can often cause the open or the short to (B+/ground) to appear.
^ The following conditions may cause a malfunction to occur:
- A voltage spike in circuit 1038.
- An open in circuit 1038.

TEST DESCRIPTION
Numbers below refer to the step numbers on the diagnostic table.
1. Always perform the heater-A/C control diagnostic system check before attempting to diagnose a DTC U1255.
2. This test checks if the DLC Diagnostic System Check was performed.
3. This test checks for a poor connection or an open in circuit 1038.
6. Always clear DTCs after a repair procedure is complete.