Service Information

Orlando

DTC U0078

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 U0078 00: Control Module Low Speed Communication Bus Off

The low speed GMLAN serial data bus is used to communicate information between the control modules. The serial data is transmitted over a single wire to the appropriate control modules. The low 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 serial data circuits more than 3 times in 5 seconds.

Action Taken When the DTC Sets

•	The control 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.

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 of 50, without a repeat of the malfunction.

Circuit/System Verification

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