U1000
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 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 5-10 seconds of beginning serial data communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.
The following modules communicate on the class 2 serial data circuit:
* The body control module (BCM)
* The communication interface module (OnStar(R))
* The digital radio receiver (DRR)
* The driver door module (DDM)
* The driver door switch (DDS)
* The driver position module (DPM)
* The fold top controller (FTC)
* The head up display (HUD)
* The HVAC control module
* The inflatable restraint sensing and diagnostic module (SDM)
* The instrument panel cluster (IPC)
* The passenger door module (PDM)
* The radio
* The remote control door lock receiver (RCDLR)
* The steering column lock control module (SCLCM)
The BCM emulates all devices that are on high speed GMLAN serial data circuits to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of the GMLAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on high speed GMLAN serial data circuits and are emulated by BCM on class 2 serial data circuit:
* The electronic brake control module (EBCM)
* The electronic suspension control module (ESC)
* The engine control module (ECM)
* The transmission control module (TCM)
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC U1000 or U1255 Class 2 Data Link
This vehicle has DTCs which include DTC Symptoms. For more information on DTC Symptoms, refer to DTC Symptom Description.
Conditions for Running the DTC
* Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
* DTCs U1300, U1301 or U1305 do not have a current status.
* The vehicle power mode requires serial data communication to occur.
Conditions for Setting the DTC
A message containing a critical operating parameter has not been received within the last 5-10 seconds after establishing class 2 serial data communication.
Action Taken When the DTC Sets
The module uses a default value for the missing parameter.
Diagnostic Aids
When a malfunction such as an open fuse to a module occurs while modules are communicating, a DTC U1001-1254 Loss of XXX Communications is set current. When the modules stop communicating the current DTC U1001-1254 Loss of XXX Communications 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 U1000 or U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 is set.
Test Description
Step 1 - Step 6:
Step 7 - Step 15:
The numbers below refer to the step numbers on the diagnostic table.
2. A DTC U1001 - U1254 "Lost Communications with XXX" with a history status may indicate the cause of U1000 or U1255. A non communicating GMLAN module can cause the DTC setting, besides other DTC in the range of U2105-U2175 or U0100-U0299.
3. The modules not communicating are the likely cause of U1000 or U1255. The modules that are available on the class 2 serial data circuit are listed in the Circuit Description.
8. The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set DTC U1001 - U1254 "Lost Communications with XXX" for those modules that it was monitoring.
10. The modules which can communicate indicate the module which cannot communicate. You must clear the serial data communication DTCs from these modules to avoid future misdiagnosis.
12. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
13. If all modules are communicating, the module which set U1000 or U1255 is the likely cause of the malfunction.