Diagnostic Aids
- 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're 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.
- 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 the mid speed GMLAN serial data system inoperative. This condition is due to the BCM using the high, mid, and low speed GMLAN systems.
- An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
- The engine will not start when there is a total malfunction of the mid speed GMLAN serial data bus.
- Technicians may find various Local Area Network communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of one or more of the following concerns:
- Vehicle will not crank
- Vehicle cranks but will not start
- Vehicle stability enhancement system warning lights and messages
- PRNDL gear indicator position errors