• 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 high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. The serial data is transmitted on two twisted wires that allow speeds up to 500 kbit/s. The twisted pair is terminated with two 120 Ω resistors, one is internal to the K20 Engine Control Module (ECM) and the other can be a separate resistor in a connector assembly or in another device. The resistors are used as the load for the High Speed GMLAN bus during normal vehicle operation. The high speed GMLAN is a differential bus. The high speed GMLAN serial data bus (+) and high speed GMLAN serial data (−) are driven to opposite extremes from a rest or idle level of approximately 2.5 V. Driving the lines to their extremes, adds 1 V to the high speed GMLAN serial data bus (+) circuit and subtracts 1 V from the high speed GMLAN serial data bus (−) circuit. If serial data is lost, devices will set a no communication code against the non-communicating device. Note that a loss of serial data DTC does not represent a failure of the device that set it.

• Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U code present. However, there is no associated “current” or “active” status. Loss of communication U codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
• A device on the data communication circuit was disconnected while the communication circuit is awake.
• Power to one or more devices was interrupted during diagnosis.
• A low battery condition was present, so some devices stop communicating when battery voltage drops below a certain threshold.
• Battery power was restored to the vehicle and devices on the communication circuit did not all re-initialize at the same time.
• If a loss of communication U code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
• Do not replace a device reporting a U code. The U code identifies which device needs to be diagnosed for a communication issue.
• Communication may be available between some devices and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to those devices using multiple serial data communication systems.
• An open in the X84 Data Link Connector ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
• The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
• Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no high speed GMLAN communications with the scan tool.
• These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of one or more of the following concerns:
• Vehicle will not crank
• Vehicle cranks but will not start
• Vehicle stability enhancement system warning lights and messages
• PRNDL gear indicator position errors
• Some devices may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no device output voltage circuit is shorted to ground before replacing the device.

• 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

Perform Circuit/System Verification using one of the following two methods:

1. Using schematics and vehicle build information, identify all high speed GMLAN devices the tested vehicle is equipped with, terminating resistors, and each device location on the high speed GMLAN bus.
2. Ignition ON/Vehicle In Service Mode.
3. From Data Circuit input selection, select Terminal 6, 14 to test the high speed GMLAN serial data bus.
4. Start the test and verify the Detected State parameter is displayed as OK.
• If the Detected State parameter is displayed as Control Module Not Awake

  Refer to Circuit/System Testing – Testing the Device Circuits.

• If the Detected State parameter is displayed as Control Module Ground Connection Malfunction

  Refer to Circuit/System Testing – Testing the Device Circuits.

• If the Detected State parameter is displayed as CAN Bus [+] (pin 6) Circuit Shorted to High Voltage or CAN Bus [-] (pin 14) Circuit Shorted to High Voltage

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short to Voltage.

• If the Detected State parameter is displayed as CAN Bus [+] (pin 6) Circuit Shorted to Low Voltage or CAN Bus [-] (pin 14) Circuit Shorted to Low Voltage

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short to Ground.

• If the Detected State parameter is displayed as CAN Bus [-] Shorted to Bus [+]

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short between the Circuits.

• If the Detected State parameter is displayed as CAN Bus Open with one or more non responding high speed GMLAN devices

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for an Open/High Resistance (Data Bus Diagnostic Tool).

• If the Detected State parameter is displayed as CAN Bus Open and all high speed GMLAN devices are responding

  Repair the open/high resistance in the circuit between the external terminating resistor and the closet responding device.

• If the Detected State parameter is displayed as OK and none of the high speed GMLAN devices are responding

  Repair the open/high resistance in the circuit between the X84 Data Link Connector and the first splice/device in the serial data circuit. If the circuit tests normal, check the MDI cable and the connection at X84 Data Link Connector prior to replacing the first device or splice pack.

• If the Detected State parameter is displayed as OK and one or more high speed GMLAN devices are not responding

  Refer to Circuit/System Testing – Testing the Device Circuits to test the non responding device closest to a responding device.

• If the Detected State parameter is displayed as OK and all high speed GMLAN devices are responding
5. All OK.

1. Ignition ON/Vehicle In Service Mode.
2. Verify two or more devices are not communicating on the high speed GMLAN serial data circuit. Refer to Data Link References to determine how many devices should be communicating on the high speed GMLAN bus.
• If only one device is not communicating

  Refer to Circuit/System Testing – Testing the Device Circuits.

• If two or more devices are not communicating
3. 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. Disconnect the scan tool from the X84 Data Link Connector. The following tests will be done at the X84 Data Link Connector.
4. Test for less than 10 Ω between the ground circuit terminal 5 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
5. Ignition ON/Vehicle In Service Mode.
6. Test for less than 4.5 V between the serial data circuits listed below and ground.
• Terminal 6
• Terminal 14
• If 4.5 V or greater

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short to Voltage.

• If less than 4.5 V
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. It may take up to 10 min for all vehicle systems to power down.
8. Test for greater than 100 Ω between the serial data circuits listed below and ground.
• Terminal 6
• Terminal 14
• If 100 Ω or less

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short to Ground.

• If greater than 100 Ω
9. Test for 50–70 Ω between the serial data circuit terminals 6 and 14.
• If less than 35 Ω

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short between the Circuits.

• If between 35–50 Ω

  There may be a third terminating resistor between the serial data circuits. This can happen if the incorrect device is installed. Some devices are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness. Refer to Circuit/System Testing – Testing the Serial Data Circuits for a Short between the Circuits.

• If greater than 70 Ω but less than infinite

  Refer to Circuit/System Testing – Testing the Serial Data Circuits for an Open/High Resistance (Digital Multimeter).

• If infinite resistance

  Repair the open/high resistance in the circuit between the X84 Data Link Connector and the first splice/device in the serial data circuit.

• If between 50–70 Ω
10. Refer to Circuit/System Testing – Testing the Device Circuits.

1. Ignition/Vehicle OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible device, 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. Ignition/Vehicle OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted to voltage, ignition ON/Vehicle In Service Mode.
4. 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
5. Repeat step 3 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 10 min for all vehicle systems to power down.
2. Disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible device.
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. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted to ground.
5. Test for greater than 100 Ω between each serial data circuit at the device connector that was just disconnected and ground.
• If both serial data circuits are 100 Ω or greater

  Replace the device that was disconnected.

• If any serial data circuit is less than 100 Ω
6. Repeat step 4 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.
• A short to ground on the serial data circuit between the X84 Data Link Connector and the first device or splice pack.

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 10 min for all vehicle systems to power down.
2. Disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible device that is not communicating.
3. 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
4. Connect the harness connectors at the device that was disconnected.
5. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another device, in the direction of the circuit shorted together.
6. 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
7. Repeat step 4 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.
• Serial data circuits shorted together between the X84 Data Link Connector and the first device or splice pack.
• 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 10 min for all vehicle systems to power down.
2. Disconnect the harness connectors with the high speed GMLAN serial data circuits at the non responding device closest to the responding device.
3. Disconnect the harness connectors with the high speed GMLAN serial data circuits at the responding device closest to the non responding device.
4. Test for less than 2 Ω in each of the high speed GMLAN serial data circuits end to end between the two devices that was just disconnected.
• If 2 Ω or greater

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

• If less than 2 Ω
5. Replace the non responding device that was disconnected.

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 10 min for all vehicle systems to power down.
2. Disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible device that is not communicating.
3. Test for less than 130 Ω between each pair of 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
4. Connect the harness connectors at the device that was disconnected.
5. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another device, in the direction of the circuit with the open/high resistance.
6. Test for less than 130 Ω between each pair of 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
7. Repeat step 4 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 10 min for all vehicle systems to power down.
2. Disconnect the harness connectors at a high speed GMLAN device that is not communicating.
3. 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
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 disconnected device.
• If the test lamp illuminates
5. Ignition ON/Vehicle In Service Mode.
6. 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
7. 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
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 less than 2 Ω in each of the high speed GMLAN serial data circuits end to end between the device harness connector and the appropriate X84 Data Link Connector terminals listed below:
• Terminal 6
• Terminal 14
• If 2 Ω or greater

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

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

Perform the Diagnostic Repair Verification after completing the repair.

Control Module References for device replacement, programming and setup