• 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.

Devices connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices. The devices have programmed information about what messages are needed to be exchanged on the serial data circuits. The messages are also supervised and some periodic messages are used by the receiver device as an availability indication of the transmitter device.

• Supply voltage to the devices is in the normal operating range.
• The vehicle power mode requires serial data communication to occur.
• The DTC U2100 does not have a current status.

A supervised periodic message that includes the transmitter device availability has not been received.

The device uses a default value for the missing parameter.

• 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.

Diagnosis of this DTC is accomplished via the symptom or an additional DTC. Refer to Scan Tool Does Not Communicate with Low Speed GMLAN Device or Diagnostic Trouble Code (DTC) List - Vehicle.