• 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.

For symptom byte information, refer to Symptom Byte List.

The low speed GMLAN serial data bus is used to communicate information between the low speed GMLAN devices. The serial data is transmitted over a single wire to the appropriate devices.

• Supply voltage to the devices are in the normal operating range.
• The vehicle power mode requires serial data communications.

The device setting the DTC has attempted to establish communications on the serial data circuits more than 3 times in 5 s.

• 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.

• 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 of 50, without a repeat of the malfunction.

1. Ignition ON/Vehicle In Service Mode.
2. Verify DTC U0078 is not set.
• If DTC U0078 is set

  Refer to Scan Tool Does Not Communicate with Low Speed GMLAN Device.

• If DTC U0078 is not set
3. All OK