* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no
associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are
transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically
after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
- A control module on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more modules was interrupted during diagnosis.
- A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above.
These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
* A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between
control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a
control module based only on a history U-code.
* Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication
issue.
* Communication may be available between the BCM and the scan tool with either the low or high speed GMLAN serial data system
inoperative. This condition is due to the BCM using both the low and high speed GMLAN systems.
* Use to determine if the module uses high or low speed GMLAN serial data communications.
* Some control modules may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and
the B+ or ignition circuit is not shorted to ground, ensure none of the control circuits are shorted to ground before replacing the control
module.
* This diagnostic can be used for any control module that is not communicating, regardless of the type of serial data circuit it is connected to,
providing the vehicle is equipped with the control module.
Reference Information
Schematic Reference
* Data Communication Schematics (See: Powertrain Management/Computers and Control Systems/Information Bus/Diagrams/Electrical
Diagrams)
* Antilock Brake System Schematics (See: Brakes and Traction Control/Antilock Brakes / Traction Control Systems/Diagrams/Electrical
Diagrams) for the ABS - Hybrid
* Antilock Brake System Schematics (See: Brakes and Traction Control/Antilock Brakes / Traction Control Systems/Diagrams/Electrical
Diagrams) for the ABS - Non-Hybrid Less than 3900 kg (8600 lb) GVW
* Antilock Brake System Schematics (See: Brakes and Traction Control/Antilock Brakes / Traction Control Systems/Diagrams/Electrical
Diagrams) for the ABS - Non-Hybrid Greater than/Equal to 3900 kg (8600 lb) GVW
Connector End View Reference
Component Connector End Views (See: Diagrams/Connector Views)
Description and Operation
Data Link Communications Description and Operation (See: Powertrain Management/Computers and Control Systems/Information
Bus/Description and Operation)
Electrical Information Reference
* Circuit Testing (See: Testing and Inspection/Component Tests and General Diagnostics/General Electrical Diagnostic Procedures/Circuit
Testing/Circuit Testing)
* Connector Repairs (See: Testing and Inspection/Component Tests and General Diagnostics/General Electrical Diagnostic
Procedures/Connector Repairs/Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (See: Testing and Inspection/Component Tests and General Diagnostics/General
Electrical Diagnostic Procedures/Circuit Testing/Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (See: Testing and Inspection/Component Tests and General Diagnostics/General Electrical Diagnostic Procedures/Wiring
Repairs/Wiring Repairs)
Scan Tool Reference
Control Module References (See: Testing and Inspection/Programming and Relearning) for scan tool information
Circuit/System Testing
1. Ignition OFF, disconnect the harness connector of the yaw rate sensor.
2. Ignition ON, verify that a test lamp illuminates between the module voltage circuit and ground.
锟斤拷If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the
EBCM.
3. Ignition OFF, test for less than 5 ohms between the module ground circuits and ground.
