• 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 serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices. The devices have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver device as an availability indication of the transmitter device. Each message contains the identification number of the transmitter device.

The system voltage is between 9–16 V.

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

Specific subsystems will not function.

• 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.

• Data Communication Schematics
• Control Module References

Master Electrical Component List

Data Link Communications Description and Operation

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

Control Module References for scan tool information

1. Ignition ON/Vehicle In Service Mode.
2. Verify that DTC U0073, U0074, U0075, U0076, U0077, U0078, U007A, U007B, U007C, U007D, P06E4, P1EB9, U1814, U2099, B1325, B1330, B1370, B1380, B1424, B1428, B1440, B1441, B1517, C0800, C0899, C12E1, P0560, or P0562 is not set.
• If any of the DTCs are set

  Refer to Diagnostic Trouble Code (DTC) List - Vehicle.

• If none of the DTCs are set
3. Refer to Circuit/System Testing.

1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at the K20 Engine Control Module. It may take up to 10 min for all vehicle systems to power down.
2. Test for less than 10 Ω between each ground circuit terminal and ground.
• If 10 Ω or greater
1. Ignition/Vehicle OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 10 Ω
3. Ignition ON/Vehicle In Service Mode.
4. If equipped, verify a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for less than 2 Ω in the B+ circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for infinite resistance between the B+ circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the K20 Engine Control Module.
• If the test lamp illuminates
5. If equipped, verify a test lamp illuminates between each ignition circuit terminal and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for less than 2 Ω in the ignition circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, verify the fuse is OK and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition/Vehicle OFF, remove the test lamp.
2. Test for infinite resistance between the ignition circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the K20 Engine Control Module.
• If the test lamp illuminates
6. If equipped, verify a test lamp illuminates between the accessory wakeup serial data circuit terminal and ground.
• If the test lamp does not illuminate
1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K9 Body Control Module.
2. Test for less than 2 Ω in the accessory wakeup serial data circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp illuminates
7.  Test for less than 4.5 V between each high speed GMLAN serial data circuit terminal and ground.
• If 4.5 V or greater

  Refer to Scan Tool Does Not Communicate with High Speed GMLAN Device to test for a short to voltage.

• If less than 4.5 V
8. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
9. Test for greater than 100 Ω between each high speed GMLAN serial data circuit terminal and ground.
• If 100 Ω or less

  Refer to Scan Tool Does Not Communicate with High Speed GMLAN Device to test for a short to ground.

• If greater than 100 Ω
10. Disconnect the harness connectors at the K56 Serial Data Gateway Module.
11. Test for less than 2 Ω in each serial data circuits end to end between the K20 Engine Control Module and the K56 Serial Data Gateway Module.
• If 2 Ω or greater

  Repair the open/high resistance in the serial data circuit.

• If less than 2 Ω
12. Replace the K20 Engine Control Module.

Perform the Diagnostic Repair Verification after completing the repair.

Control Module References for device replacement, programming and setup