Service Information

Volt

DTC U0077

Diagnostic Instructions

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

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

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

DTC Descriptor

DTC U0077 00: Control Module Chassis Expansion Communication Bus Off

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 control modules are in the normal operating range.

•	The vehicle power mode requires serial data communications.

Conditions for Setting the DTC

The control module 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 module suspends all message transmission.

•	The control module uses default values for all parameters received on the serial data circuits.

•	The control module 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 module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

•	DTC U0077 is a type A DTC.

Circuit/System Verification

Diagnosis is accomplished using the symptom. Refer to Scan Tool Does Not Communicate with Chassis High Speed GMLAN Device .