• 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 devices connected to the high speed GMLAN serial data circuits (circuit numbers 2500 & 2501) monitor for serial data communications during normal vehicle operation. The high speed GMLAN serial data bus uses terminating resistors that are in parallel with the high speed GMLAN (+) and (−) circuits.

• Supply voltage to the devices is 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.
• In the transmission control module, DTC U0073 will cause the transmission to go into default gears.
• In the engine control module and transmission control module, DTC U0073 will cause the malfunction indicator lamp (MIL) to illuminate.
• If equipped with eAssist, DTC U0073 in the hybrid powertrain control module is a type B DTC and will cause the malfunction indicator lamp to illuminate and result in the engine operating in conventional engine mode and will maintain 14 V module operation.
• The device inhibits the setting of all other communication DTCs.

• The engine control module or transmission control module turns OFF the MIL after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
• If equipped with eAssist, the hybrid powertrain control module turns off the MIL after the diagnostic runs and does not fail during subsequent ignition cycles. Normal operation will resume 5 s after subsequent ignition cycle.
• 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. Refer to Data Link References to determine which serial data communication system is used for a specific device.
2. This DTC may not be retrieved with a current status. Diagnosis is accomplished using the symptom. Refer to Symptoms - Data Communications.