Test Description
The numbers below refer to the step numbers on the diagnostic table.
A Lost Communication with XXX DTC with a history status may indicate the cause of U1000 or U1255.
The module which is not communicating is the likely cause of U1000 or U1255 being set. The following modules communicate on the class 2 serial data circuit
- HVAC Control Module (HCM)
- Suspension Control Module (SCM)
- Transfer Case Shift Control Module (TCSCM)
- Electronic Brake Control Module (EBCM)
- Powertrain Control Module (PCM)
- OnStar® module (ONS)
- Body Control Module (BCM)
- Instrument Panel Cluster (IPC)
- Driver Information Center (DIC)
- Memory Mirror Module (MMM)
- Memory Seat Module (MSM)
- Inflatable restraint Sensing and Diagnostic Module (SDM)
The module which was not communicating may have set lost communication with XXX DTCs for those modules that it was monitoring.
The module which was not communicating may have set lost communication with XXX DTCs for those modules that it was monitoring.
The module which was not communicating may have set lost communication with XXX DTCs for those modules that it was monitoring.
The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
The module which set U1000 or U1255 is the likely cause of the malfunction.
Scheme 7
Scheme 8
The numbers below refer to the step numbers on the diagnostic table.
A module which loses power during an ignition cycle will cause other module to set lost communication DTCs.
A module which loses power during an ignition cycle will cause other module to set lost communication DTCs.
The malfunction is due to an open in the class 2 serial data circuit or an open in the module.
The module which was not communicating due to an open in the class 2 serial data circuit may have set lost communication DTCs for those modules that it was monitoring.
The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs from these modules to avoid future misdiagnosis.
Scheme 9
Scheme 10
The number below refers to the step number on the diagnostic table.
If the battery positive voltage and ground circuits of the DLC are functioning properly. The malfunction must be due to the scan tool.
Scheme 11
The numbers below refer to the step numbers on the diagnostic table.
A partial loss of communication in the class 2 serial data circuit uses a different procedure than a total loss of communication of the class 2 serial data circuit.
The following DTCs may be retrieved with a history status. These DTCs are not the cause of the present condition.
- U1300
- U1301
- U1305
A State of Health DTC with a history status may be present along with a U1000 or U1255 with a current status. This indicates that the malfunction occurred when the ignition was on.
Normal class 2 serial data communication cannot take place until the Power Mode Master (PMM) module sends the appropriate power mode message. If the PMM does not send a wake-up message, other modules on the class 2 serial data circuit may not communicate.
If there are no current DTCs that begin with the letter "U", the communication concern has been repaired.
The communication concern may have prevented diagnosis of the customer complaint.
Scheme 12
Scheme 13
Scheme 14
Data Link Communications Circuit Description & Operation
The Class 2 serial data link allows modules to transmit, receive, or exchange data between each other
- HVAC Control Module (HCM)
- Rear Wheel Steering Module (RWS)
- Suspension Control Module (SCM)
- Active Transfer Case (ATC)
- Electronic Brake Control Module (EBCM)
- Powertrain Control Module (PCM)
- Transmission Control Module (TCM)
- OnStar® Module (ONS)
- Body Control Module (BCM)
- Instrument Panel Cluster (IPC)
- Driver Information Center (DIC)
- Mirror Memory Module (MMM)
- Memory Seat Module (MSM)
- Inflatable restraint Sensing and Diagnostic Module (SDM)