• 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.

The devices connected to the gateway isolated high speed GMLAN serial data circuits (circuit numbers 1304 & 1305) monitor for serial data communications during normal vehicle operation. The K56 Serial Data Gateway Module acts as the gateway module between devices on primary high speed GMLAN bus and devices on gateway isolated high speed GMLAN bus. The gateway isolated high speed GMLAN serial data bus uses terminating resistors that are in parallel with the gateway isolated high speed GMLAN (+) and (−) circuits.

• The system voltage is between 9–16 V.
• The vehicle power mode requires serial data communications.

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

• 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, 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, U0077, U0078, U007A, 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, disconnect the harness connectors with the gateway isolated high speed GMLAN serial data circuits at an easily accessible device that is not communicating. Refer to Data Link References to determine which devices are on the gateway isolated high speed GMLAN bus.
2. Ignition ON/Vehicle In Service Mode.
3. Test for less than 4.5 V between each gateway isolated high speed GMLAN serial data circuit at the device that was just disconnected and ground.
• If 4.5 V or greater

  Refer to Testing the Serial Data Circuits for a Short to Voltage.

• If less than 4.5 V
4. 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 2 min for all vehicle systems to power down.
5. Test for greater than 100 Ω between each gateway isolated high speed GMLAN serial data circuit at the device that was just disconnected and ground.
• If 100 Ω or less

  Refer to Testing the Serial Data Circuits for a Short to Ground.

• If greater than 100 Ω
6. Test for 110–130 Ω between each pair of the gateway isolated high speed GMLAN serial data circuits at the device that was just disconnected:
• If less than 110 Ω

  Refer to Testing the Serial Data Circuits for a Short between the Circuits.

• If greater than 130 Ω

  Refer to Testing the Serial Data Circuits for an Open/High Resistance.

• If between 110–130 Ω
7. Refer to Testing the Device Circuits.

1. Ignition/Vehicle OFF, disconnect the harness connectors with the gateway isolated high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted to voltage, ignition ON/Vehicle In Service Mode.
2. Test for greater than 4.5 V between each serial data circuit at the device connector that was just disconnected and ground.
• If each serial data circuit is 4.5 V or less
1. Ignition/Vehicle OFF.
2. Test for less than 10 Ω between each of the device's ground circuit terminals and ground.
• If 10 Ω or greater, repair the open/high resistance in the circuit.
• If less than 10 Ω, replace the device that was disconnected.
• If any serial data circuit is greater than 4.5 V
3. Repeat step 1 until one of the following conditions are isolated:
• A short to voltage on the serial data circuit between two devices or splice packs, if equipped.
• A short to voltage on the serial data circuit between a device and a terminating resistor.

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. It may take up to 2 min for all vehicle systems to power down.
2. Disconnect the harness connectors with the gateway isolated high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted to ground.
3. Test for greater than 100 Ω between each serial data circuit at the device connector that was just disconnected and ground.
• If each serial data circuit is 100 Ω or greater

  Replace the device that was disconnected.

• If any serial data circuit is less than 100 Ω
4. Repeat step 1 until one of the following conditions are isolated:
• A short to ground on the serial data circuit between two devices or splice packs, if equipped.
• A short to ground on the serial data circuit between a device and a terminating resistor.

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. It may take up to 2 min for all vehicle systems to power down.
2. Connect the harness connectors at the device that was previously disconnected.
3. Disconnect the harness connectors with the gateway isolated high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted together.
4. Test for greater than 110 Ω between each pair of serial data circuits at the device connector that was just disconnected.
• If each pair of serial data circuits is 110 Ω or greater

  Replace the device that was disconnected.

• If any pair of serial data circuits is less than 110 Ω
5. Repeat step 2 until one of the following conditions are isolated:
• Serial data circuits shorted together between two devices or splice packs, if equipped.
• Serial data circuits shorted together between a device and a terminating resistor.
• A shorted terminating resistor.

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. It may take up to 2 min for all vehicle systems to power down.
2. Connect the harness connectors at the device that was previously disconnected.
3. Disconnect the harness connectors with the gateway isolated high speed GMLAN serial data circuits at another device, in the direction of the circuit with the open/high resistance.
4. Test for less than 130 Ω between each pair of the serial data circuits at the device connector that was just disconnected.
• If each pair of serial data circuits is 130 Ω or less

  Replace the device that was disconnected.

• If any pair of serial data circuits is greater than 130 Ω
5. Repeat step 2 until one of the following conditions are isolated:
• An open/high resistance on the serial data circuit between two devices or splice packs, if equipped.
• An open/high resistance on the serial data circuit between a device and a terminating resistor.
• An open/high resistance terminating resistor.

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. It may take up to 2 min for all vehicle systems to power down.
2. Test for less than 10 Ω between each ground circuit terminal at the device that was just disconnected 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. 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 disconnected device.
• If the test lamp illuminates
4. Ignition ON/Vehicle In Service Mode.
5. If equipped, verify a test lamp illuminates between each ignition circuit terminal, which has a fuse in the circuit, 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 disconnected device.
• If the test lamp illuminates
6. If equipped, verify a test lamp illuminates between each ignition circuit terminal, which is controlled by a control module, and ground.
• If the test lamp does not illuminate
1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the control module that controls the ignition circuit.
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
3. 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, replace the control module that controls the ignition circuit.
• If the test lamp illuminates
7. 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 K56 Serial Data Gateway Module. It may take up to 2 min for all vehicle systems to power down.
8. Test for less than 2 Ω in each of the gateway isolated high speed GMLAN serial data circuits end to end between the non communicating device 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
9. Test for less than 2 Ω in each of the primary high speed GMLAN serial data circuits (circuit numbers 2500 & 2501) end to end between the K56 Serial Data Gateway Module and the appropriate X84 Data Link Connector (DLC) terminals listed below.
• X84 Data Link Connector high speed GMLAN serial data circuit terminal 6
• X84 Data Link Connector high speed GMLAN serial data circuit terminal 14
• If 2 Ω or greater

  Repair the open/high resistance in the primary high speed GMLAN serial data circuit.

• If less than 2 Ω
10. Replace the non communicating device that was disconnected.

Perform the Diagnostic Repair Verification after completing the repair.

Control Module References for device replacement, programming and setup