U0077

DTC U0077

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.

* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC U0077
- Control Module Communication Chassis Expansion CAN Bus Off

For symptom byte information, refer to Symptom Byte List Symptom Byte List.

Circuit/System Description

The GMLAN high speed chassis expansion bus functions the same as the GMLAN high speed bus, and the two buses operate in parallel. The expansion chassis bus was added to reduce message congestion on the primary high speed bus. Since the GMLAN high speed chassis bus and primary GMLAN high speed bus operate in the same manner, the diagnostics for each are the same.

The chassis high speed GMLAN serial data circuits also connect directly to the data link connector (DLC).

Conditions for Running the DTC

* Supply voltage to the devices are in the normal operating range.

* The vehicle power mode requires serial data communications.

Conditions for Setting the DTC

The device setting the DTC has attempted to establish communications on the chassis high speed GMLAN serial data circuits more than 3 times.

Action Taken When the DTC Sets

* The device suspends all message transmission.

* The device uses default values for all parameters received on the serial data circuits.

* The device inhibits the setting of all other communication DTCs.

* DTC U0077 is a type A DTC.

Conditions for Clearing the DTC

* A current DTC clears when the malfunction is no longer present.

* A history DTC clears when the device ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

* DTC U0077 is a type A DTC.

Circuit/System Verification

1. Vehicle in Service Mode.

2. Verify DTC U0077 is not set.

• If DTC U0077 is set
Refer to Scan Tool Does Not Communicate with Chassis High Speed GMLAN Device Scan Tool Does Not Communicate with Chassis High Speed GMLAN Device.

• If DTC U0077 is not set

3. All OK