U1255

DTC U1000 and U1255

Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for state of health (SOH) monitoring. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an identification number with at least one critical parameter within about 5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. If more than one critical parameter does not have an identification number association, the DTC will only be reported once.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

* DTC U1000 Class 2 Data Link Malfunction
* DTC U1255 Lost Communications

Conditions for Running the DTC

* Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
* Diagnostic trouble codes U1300, U1301, and U1305 do not have a current status.
* The module operation or the vehicle power mode requires serial data communication to occur.

Conditions for Setting the DTC

At least one critical operating parameter has not been associated with an identification number within about 5 seconds after beginning serial data communication.

Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

Conditions for Clearing the DTC

* A current DTC U1000 will clear when all critical operating parameters for the module have been associated with an identification number, or at the end of the current ignition cycle.
* A history DTC U1000 will clear after 50 ignition switch ON/OFF cycles with no repeat of the failure.

Diagnostic Aids

When a malfunction, such as an open fuse to a module occurs while modules are communicating, a Lost Communication DTC for a specific module is set as a current DTC. When the modules stop communicating, the current Lost Communication DTC is cleared, but the history DTC remains. When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules, so DTC U1000 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only DTC U1000 or U1255 is set.

Test Description

Step 1 - Step 12:

Step 13 - Step 15:

The numbers below refer to the step numbers on the diagnostic table.
1. A Lost Communication with XXX DTC with a history status may indicate the cause of U1000.
2. The module that is not communicating is the likely cause of U1000 or U1255 being set.
5. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
6. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
7. The module that was not communicating may have set Lost Communication with XXX DTCs for those modules that the module was monitoring.
11. The modules that communicate indicate the module that cannot communicate. You must clear the DTC from the modules in order to avoid future misdiagnosis.
13. If all modules are communicating, the module that set U1000 may have done so due to some other condition.
14. The module that set U1000 is the likely cause of the malfunction.