Data Communications System: Diagnosis GMC Terrain I

Diagnostic Code Index

DIAGNOSTIC CODE INDEX

Diagnostic Instructions

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Conditions for Running the DTC

The module runs the program to detect an internal fault when power up is commanded. The only requirements are voltage and ground. This program runs even if the voltage is out of the valid operating range.

Conditions for Setting the DTC

The module has detected an internal malfunction.

Action Taken When the DTC Sets

The module refuses all additional inputs.

Conditions for Clearing the DTC

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

Diagnostic Aids

1. This DTC may be stored as a history DTC without affecting the operation of the module.
2. If stored only as a history DTC and not retrieved as a current DTC, do not replace the module.
3. If this DTC is retrieved as both a current and history DTC, replace the module that set the DTC.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Battery voltage is between 9-16 V and data link communications operate normally.

The control module is not configured properly.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. The control module runs the program to detect an internal fault when power up is commanded. The only requirements are voltage and ground. This program runs even if the voltage is out of the valid operating range.
2. The keyless entry control module will set this DTC with symptom byte 39 when the keyless entry control module antenna is activated.

The control module has detected an internal malfunction.

The control module refuses all additional inputs.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the control module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. If DTC B101D is set as history or current in the K36 Inflatable Restraint Sensing and Diagnostic Module (SDM) and there are Air Bag or Pretensioner DTC's with symptom byte 02, it is important to repair the deployment loop issue first prior to replacing the K36 SDM. Failure to repair the Air Bag or Pretensioner wiring concern first may cause B101D to reset in the replacement K36 SDM.
2. This DTC may be stored as a history DTC without affecting the operation of the control module.
3. Do not replace a control module based only on DTC B101D being set in history with the exception of the following control modules: K36 Inflatable Restraint Sensing and Diagnostic Module (SDM) K85 Passenger Presence Detection Module
4. If DTC B101D is set as current, replace the appropriate control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Battery voltage is between 9-16 V and data link communications operate normally.

The control module is not configured properly.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

The module runs the program to detect an internal fault when power up is commanded. The only requirements are voltage and ground. This program runs even if the voltage is out of the valid operating range.

The module has detected an internal malfunction.

The module refuses all additional inputs.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. This DTC may be stored as a history DTC without affecting the operation of the module.
2. If stored only as a history DTC and not retrieved as a current DTC, do not replace the module.
3. If this DTC is retrieved as both a current and history DTC, replace the module that set the DTC.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

The module runs the program to detect an internal fault when power up is commanded. The only requirements are voltage and ground. This program runs even if the voltage is out of the valid operating range.

The module has detected an internal malfunction.

The module refuses all additional inputs.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. This DTC may be stored as a history DTC without affecting the operation of the module.
2. If stored only as a history DTC and not retrieved as a current DTC, do not replace the module.
3. If this DTC is retrieved as both a current and history DTC, replace the module that set the DTC.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Battery voltage is between 9-16 V and data link communications operate normally.

The control module is not configured properly.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. The ignition is ON.
2. The system voltage is greater than 9.5 V.

The control module detects an internal malfunction or incomplete programming.

DTCs P0601, P0602, P0603, P0604, P0606, and P062F are Type A DTCs.

DTCs P0601, P0602, P0603, P0604, P0606, and P062F are Type A DTCs.

If stored only as a history DTC and not retrieved as a current DTC, do not replace the control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Circuit Description

Modules connected to the GMLAN serial data circuits monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. Each module on GMLAN network maintains a transmit error counter (TEC) and a receive error counter (REC). The counter values increase with detected errors and will decrease with error-free messages. If the TEC value exceeds 255 the controller removes itself from the network and a DTC U0001 will be set.

1. Voltage supplied to the module is in the normal operating voltage range.
2. The vehicle power mode requires serial data communication to occur.

A certain number of no valid transmitted messages on the GMLAN serial data circuits are detected by the module.

1. The module suspends all message transmission.
2. The module uses default values for all parameters received on the GMLAN serial data circuits.
3. The module inhibits the setting of all other GMLAN communication DTCs.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. This DTC cannot be retrieved with a current status. Diagnosis of current DTC is accomplished via the symptom, Scan Tool Does Not Communicate with GMLAN Device. Refer to «Scan Tool Does Not Communicate with High Speed GMLAN Device»(ref-491161-S24310616442012073000000) .
2. An intermittent condition is likely to be caused by a short on the GMLAN serial data circuits. Use the Scan Tool Does Not Communicate with GMLAN Device procedure in order to isolate an intermittent condition. Refer to «Scan Tool Does Not Communicate with High Speed GMLAN Device»(ref-491161-S24310616442012073000000) .

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. Voltage supplied to the modules is in the normal operating voltage range.
2. The vehicle power mode requires serial data communication to occur.
3. The DTC U0001 does not have a current status.

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

The control module uses a default value for the missing parameter.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

Circuit/System Testing

1. With a scan tool, verify DTC U0100-U02FF or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U02FF»(ref-491161-S42425685932012073000000) , or «DTC U2105-U2199»(ref-491161-S25617731092012073000000) .
2. Ignition OFF, disconnect the applicable control module harness connector.
3. Ignition OFF, test for less than 5 ohms between the applicable control module ground circuit and ground. If greater than the specified range, test the circuit for an open/high resistance.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable control module and ground: The battery positive voltage input circuits The battery positive voltage output circuits The ignition voltage input circuits The ignition voltage output circuits The switched battery positive voltage supply circuits If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuit fuse is open, test the circuit for a short to ground. If the circuit tests normal, replace the applicable control module.
5. Ignition OFF, test for less than 5 ohms between the non-communicating control module GMLAN serial data circuits and the control module reporting DTC U0002. If greater than the specified range, test the circuit for an open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Conditions for Running the DTCs

1. Voltage at the modules is in the normal operating voltage range.
2. The vehicle power mode requires serial data communication to occur.
3. The DTC U2100 does not have a current status.

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

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. Supply voltage at the control modules is in the normal operating range.
2. The vehicle power mode requires serial data communications.

The control module setting the DTC has attempted to establish communications on the serial data circuits more than 3 times in 5 seconds.

1. The ECM/TCM modules turn off the MIL after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
2. 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 seconds after subsequent ignition cycle.
3. A current DTC clears when the malfunction is no longer present.
4. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Diagnostic Fault Information

The system voltage is between 9-16 V.

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

Specific subsystems will not function.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. Use the «Data Link References»(ref-491161-S20509971852012073000000) to identify the mid speed or chassis high speed GMLAN serial data modules.
2. Some control modules 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 control module output voltage circuit is shorted to ground before replacing the control module.
3. An open in the DLC ground circuit terminal 5 will allow the scan tool to operate to set up the vehicle on the tool and then not communicate with the vehicle.

Note. Each control module may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following: Control modules the vehicle is equipped with Mid speed or chassis high speed serial data circuit terminating resistors Control module locations on the serial data circuits Each control module's mid speed or chassis high speed serial data circuit terminals

1. Refer to «Data Link References»(ref-491161-S20509971852012073000000) to determine if the control module setting DTC U0074 is on the mid speed or chassis high speed serial data circuits. Only diagnose the affected serial data circuits.
2. Attempt to communicate with all control modules on the mid speed or chassis high speed serial data circuit. Refer to «Data Link References»(ref-491161-S20509971852012073000000) . Communications should not be available with two or more control modules on the mid speed or chassis high speed serial data circuit. If only one control module is not communicating, diagnose that control module only. Refer to «DTC U0100-U02FF»(ref-491161-S42425685932012073000000) .
3. Disconnect the scan tool from the X84 DLC. The following tests will be done at the X84 DLC connector.
4. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 10 ohms between the ground circuit terminal 5 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
5. Ignition ON, test for less than 4.5 V between each serial data circuit listed below and ground: Mid speed GMLAN serial data terminal 3 Mid speed GMLAN serial data terminal 11 Chassis high speed GMLAN serial data terminal 12 Chassis high speed GMLAN serial data terminal 13 If greater than the specified range, test the serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuits for a Short to Voltage below.
6. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 100 ohms between each serial data circuit listed below and ground: Mid speed GMLAN serial data terminal 3 Mid speed GMLAN serial data terminal 11 Chassis high speed GMLAN serial data terminal 12 Chassis high speed GMLAN serial data terminal 13 If less than the specified range, test the serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to Ground below.
7. Test for 50-70 ohms between the serial data circuits listed below: Mid speed GMLAN serial data circuits terminal 3 and terminal 11 Chassis high speed GMLAN serial data circuits terminal 12 and terminal 13 If less than 35 ohms, test for a short between the serial data circuits. Refer to Testing the Serial Data Circuits for a Short between the Circuits below. If 35-50 ohms there may be a third terminating resistor between the serial data circuits. This can happen if the incorrect control module is installed. Some control modules are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness. If greater than 70 ohms but less than infinite, test the serial data circuit for an open/high resistance. Refer to Testing the Serial Data Circuits for an Open/High Resistance below. If infinite, test the serial data circuits between the DLC and the first connection to the serial data circuit for an open/high resistance.
8. If the circuits test normal, refer to Testing the Control Module Circuits below.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 4.5 V between each serial data circuit and ground. Verify that one or more serial data circuits are greater than 4.5 V. If all serial data circuits are less than the specified range, replace the control module that was disconnected.
3. Ignition OFF, disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the circuit shorted to voltage.
4. Ignition ON, test for greater than 4.5 V between each serial data circuit and ground. Verify that one or more serial data circuits are greater than 4.5 V. If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated: A short to voltage on the serial data circuit between 2 control modules A short to voltage on the serial data circuit between a control module and a terminating resistor

Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module.
2. Test for less than 100 ohms between each serial data circuit and ground. Verify that one or more serial data circuits are less than 100 ohms. If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
3. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the circuit shorted to ground.
4. Test for less than 100 ohms between each serial data circuit and ground. Verify that one or more serial data circuits are less than 100 ohms. If all serial data circuits are greater than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated: A short to ground on the serial data circuit between 2 control modules A short to ground on the serial data circuit between a control module and a terminating resistor

Testing the Serial Data Circuits for a Short between the Circuits

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module, that is not communicating.
2. Test for less than 110 ohms between each pair of serial data circuits. Verify that one pair of serial data circuits are less than 110 ohms. If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the circuits shorted together.
5. Test for less than 110 ohms between each pair of serial data circuits. Verify that one pair of serial data circuits are less than 110 ohms. If each pair of serial data circuits is greater than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated: Serial data circuits shorted together between 2 control modules Serial data circuits shorted together between a control module and a terminating resistor A shorted terminating resistor

Testing the Serial Data Circuits for a Open/High Resistance

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module, that is not communicating.
2. Test for greater than 130 ohms between each pair of serial data circuits. Verify that one pair of serial data circuits are greater than 130 ohms. If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the open circuit.
5. Test for greater than 130 ohms between each pair of serial data circuits. Verify that one pair of serial data circuits are greater than 130 ohms. If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
6. Repeat step 3 until one of the following conditions are isolated: An open/high resistance on the serial data circuit between 2 control modules An open/high resistance on the serial data circuit between a control module and a terminating resistor An open/high resistance terminating resistor

Testing the Control Module Circuits

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors at an easily accessible control module that is not communicating.
2. Test for less than 10 ohms between each ground circuit terminal and ground, at the control module connector that was just disconnected. If greater than the specified range, test the ground circuit and ground connection for an open/high resistance.
3. Ignition OFF, verify that a test lamp does not illuminate between each ignition circuit terminal and ground. If the test lamp does illuminate, test the ignition circuit for a short to voltage. If the circuits test normal, replace the appropriate control module.
4. Ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuits test normal, replace the appropriate control module.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 130 ohms between each pair of serial data circuits. If greater than the specified range, test the serial data circuits for an open/high resistance.
6. Connect the harness connectors at the control module that was just disconnected.
7. Repeat step 1 at another control module until a short to voltage, a short to ground, or an open/high resistance is found on the ground, ignition, or serial data circuits.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. Supply voltage to the devices are in the normal operating range.
2. 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 seconds.

1. The device suspends all message transmission.
2. The device uses default values for all parameters received on the serial data circuits.
3. The device inhibits the setting of all other communication DTCs.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the device ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

DTC Descriptor

Refer to Control Module U Code List .

The system voltage is between 9-16 V.

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

1. Specific subsystems will not function.
2. DTC U0100 in the TCM will cause the transmission to go into default gears.
3. Both DTC U0100 in the TCM and DTC U0101 in the ECM will cause the malfunction indicator lamp (MIL) to illuminate.

1. The ECM/TCM module turns OFF the MIL after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
2. A current DTC clears when the malfunction is no longer present.
3. A history DTC clears when the control module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. 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 control module on the data communication circuit was disconnected while the communication circuit is awake. Power to one or more control modules was interrupted during diagnosis. A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. Battery power was restored to the vehicle and control modules 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.
2. A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code.
3. Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
4. Communication may be available between the BCM and the scan tool with either the low or high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the low and high speed GMLAN systems.
5. Use «Data Link References»(ref-491161-S20509971852012073000000) to determine if the control module uses high or low speed GMLAN serial data communications.
6. Some control modules may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to ground, ensure none of the control circuits are shorted to ground before replacing the control module.
7. Some intermittent communication concerns may be caused by fretting corrosion on the serial data circuit terminals. Inspect all connectors at the control module that set the communication DTC, the control module that the communication DTC was set against, and any inline harness connectors between the two control modules. Do not replace a control module based only on fretting corrosion. Refer to bulletin 09-06-03-004 for assistance with the diagnosis and repair of this condition.
8. This diagnostic can be used for any control module that is not communicating, regardless of the type of serial data circuit it is connected to, providing the vehicle is equipped with the control module.

Note. Use the schematic to identify the following: Control modules the vehicle is equipped with Control module locations on the low and high speed GMLAN serial data circuits The control modules B+, ignition, ground, communication enable and serial data circuit terminals

1. Ignition OFF, disconnect the harness connector at of the control module that is not communicating.
2. Test for less than 10 ohms between each ground circuit terminal and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. If equipped, ignition ON, verify that a test lamp illuminates between each B+ circuit terminal and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the control module.
4. If equipped, ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the control module.
5. If equipped, ignition ON, verify that a test lamp illuminates between the communication enable circuit terminal OR the accessory wakeup circuit terminal and ground. If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K9 body control module, for an open/high resistance or short to ground. If the circuits test normal, replace the K9 body control module.
6. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters away from vehicle. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 5 ohms between the serial data circuit terminals and the appropriate X84 data link connector (DLC) terminal listed below: Class 2 serial data circuit terminal 2 Low speed GMLAN serial data terminal 1 High speed GMLAN serial data terminal 6 or 14 Mid speed GMLAN serial data terminal 3 or 11 Chassis high speed GMLAN serial data terminal 12 or 13 If greater than the specified range, test the serial data circuit for an open/high resistance between the non communicating control module and a module setting the DTC or a serial data splice pack.
7. Test for less than 130 ohms between each pair of high speed GMLAN serial data circuits. If greater than the specified range, test the serial data circuits for an open/high resistance.
8. If all circuits test normal, replace the control module that is not communicating.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Battery voltage is between 9-16 V and data link communications operate normally.

The control module is not configured properly.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Refer to Control Module U Code List .

Battery voltage is between 9-16 V and data link communications operate normally.

The control module is not configured properly.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Refer to Control Module U Code List .

1. A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code.
2. Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
3. Communication will be available between the master control module and the scan tool if there is a loss of communications with any of the other control modules on the LIN bus network.
4. An open in the LIN bus serial data circuit between the splice pack and a module will only affect that specific module. This type of failure will set a loss of communication DTC for each control module affected and the other control modules will still communicate.
5. Some intermittent communication concerns may be caused by fretting corrosion on the serial data circuit terminals. Inspect all connectors at the control module that set the communication DTC, the control module that the communication DTC was set against, and any inline harness connectors between the two control modules. Do not replace a control module based only on fretting corrosion. Refer to bulletin 09-06-03-004 for assistance with the diagnosis and repair of this condition..

Note. Use the schematic to identify the following: LIN devices the vehicle are equipped with The LIN device's B+, ignition, ground, and serial data circuit terminals Use the Control Module U-Code list to determine which control module is not communicating with the master control module. Refer to Control Module U Code List . A loss of serial data communications DTC does not represent a failure of the master control module that set it.

1. Ignition OFF, disconnect the harness connector at a LIN device that is not communicating.
2. Test for less than 10 ohms between each ground circuit terminal and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. If equipped, verify that a test lamp illuminates between each B+ circuit terminal and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the LIN device for a short to ground. If the circuits test normal, replace the LIN device.
4. If equipped, ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the LIN device for a short to ground. If the circuits test normal, replace the LIN device.
5. Ignition ON, test for 2-12 V between the LIN serial data circuit and ground. If greater than the specified range, test the LIN serial data circuit for a short to voltage. If the circuit tests normal, replace the master control module. If less than the specified range, test the LIN serial data circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the master control module.
6. If all circuits test normal, replace the LIN device.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. The system voltage is between 9-16 volts.
2. The vehicle power mode master requires serial data communication to occur.

The BCM senses a short to ground on the serial data communication enable circuit.

1. The output command is turned off while the malfunction is present.
2. The modules use a default value for the missing parameters until the next ignition cycle.
3. The module(s) is never signaled, therefore the specific subsystem(s) will not function.
4. The vehicle will not start while the circuit is shorted to ground.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. Use the DMM MIN/MAX function to capture/locate intermittent conditions.
2. Short to ground will also set multiple history no communication codes for GMLAN high speed modules.

Note. Use the schematic to determine which control modules are on the BCM communications enable circuit 22, X4.

1. Ignition OFF, disconnect the harness connector at the most easily accessible control module on the K9 BCM communications enable circuit.
2. Ignition ON, retest for current DTCs. DTC U1814 should remain current. If the DTC becomes history, replace the control module that was just disconnected.
3. With the prior control modules disconnected, repeat steps 1 and 2 for each control module on the circuit except the K9 BCM.
4. Ignition OFF, disconnect the X4 harness connector at the K9 BCM.
5. Test for infinite resistance between the serial data communication enable circuit terminal 22 X4 and ground. If less than infinite resistance, repair the communications enable circuit for a short to ground.
6. If all circuits tests normal, replace the K9 BCM.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. The system voltage is between 9-16 volts.
2. The vehicle power mode master requires serial data communication to occur.

1. A supervised periodic message that includes the transmitter module availability has not been received.
2. Rear differential clutch control module detects an invalid signal from Multi-Axis Acceleration Sensor Module and/or Steering Wheel Angle Sensor.

All wheel drive performance will be temporarily degraded.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. Use the DTC Descriptor list above to determine the module which is not communicating.
2. If there are multiple non-communicating modules choose the one closest to the EBCM.

Note. Use the DTC Descriptors and Diagnostic Aids above to determine the control module which is not communicating and should be tested. Use the schematics to identify the following: Control modules the vehicle is equipped with Control module locations on the Chassis high speed GMLAN serial data circuits The control module serial data circuit terminals

1. Ignition OFF, disconnect the harness connector at the non communicating control module.
2. Test for less than 5 ohms between the ground circuit terminal 6 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify that a test lamp illuminates between the ignition circuit terminal 5 and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K17 electronic brake control module (EBCM).
4. Ignition OFF, test for less than 1 ohms in each serial data circuit between the non communicating control module and the control module setting the DTC. If greater than the specified range, test the serial data circuits for an open/high resistance.
5. If all circuits test normal, replace the non communicating control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. Voltage supplied to the module is in the normal operating voltage range.
2. The vehicle power mode requires serial data communication to occur.

1. The control module is not configured properly.
2. Control unit recognises a programming error.
3. CAN-Bus configuration is invalid

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. The system voltage is between 9-16 volts.
2. The vehicle power mode master requires serial data communication to occur.

The BCM senses a short to ground on the serial data communication enable circuit.

1. The output command is turned off while the malfunction is present.
2. The control modules use a default value for the missing parameters until the next ignition cycle.
3. The control module(s) is never signaled, therefore the specific subsystem(s) will not function.
4. The vehicle will not start while the circuit is shorted to ground.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. Use the DMM MIN/MAX function to capture/locate intermittent conditions.
2. Short to ground will also set multiple history no communication codes for GMLAN high speed modules.

Note. Use the schematic to determine which control modules are on the BCM communications enable circuit 23, X4.

1. Ignition OFF, disconnect the harness connector at the most easily accessible control module on the K9 BCM communication enable circuit.
2. Ignition ON, retest for current DTCs. DTC U2099 should remain current. If the DTC becomes history, replace the control module that was just disconnected.
3. With the prior control modules disconnected, repeat steps 1 and 2 for each control module on the circuit except the K9 BCM.
4. Ignition OFF, disconnect the X4 harness connector at the K9 BCM.
5. Test for infinite resistance between the serial data communication enable circuit terminal 23 X4 and ground. If less than infinite resistance, repair the communication enable circuit for a short to ground.
6. If all circuits tests normal, replace the K9 BCM.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. Voltage supplied to the module is in the normal operating voltage range.
2. The vehicle power mode requires serial data communication to occur.

1. The control module is not configured properly.
2. Control unit recognises a programming error.
3. CAN-Bus configuration is invalid

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

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

DTC U2103 will set when fewer modules are detected on the buss before the modules can learn the other module IDs. If the communication buss is opened, or if the module power supply is lost when the vehicle is in the OFF power mode, and then placed in the RUN power mode, U2103 will set in specific modules.

1. Voltage supplied to the modules is in the normal operating voltage range.
2. The vehicle power mode requires serial data communication to occur.
3. The DTC U2100 does not have a current status.

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

The module uses a default value for the missing parameter.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Diagnosis of this DTC is accomplished via the symptom or an additional DTC. Refer to «Symptoms - Data Communications»(ref-491161-S11421992392012073000000) , or «DTC U0100-U02FF»(ref-491161-S42425685932012073000000) .
2. An intermittent condition is likely to be caused by a short on the GMLAN serial data circuits. Use the «DTC U0100-U02FF»(ref-491161-S42425685932012073000000) procedure in order to isolate an intermittent condition.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using the diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

Refer to Control Module U Code List .

1. The system voltage is between 9-16 V.
2. The vehicle power mode requires serial data communication to occur.

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

The control module uses a default value for the missing parameters.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the control module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

1. If there are multiple non-communicating modules choose the one closest to the data link connector (DLC).
2. 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 control module on the data communication circuit was disconnected while the communication circuit is awake. Power to one or more control modules was interrupted during diagnosis. A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. Battery power was restored to the vehicle and control modules 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.
3. A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code.
4. Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
5. Use «Data Link References»(ref-491161-S20509971852012073000000) to determine if the control module uses high or low speed GMLAN serial data communications
6. Some control modules 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 control module output voltage circuit is shorted to ground before replacing the control module.
7. Some intermittent communication concerns may be caused by fretting corrosion on the serial data circuit terminals. Inspect all connectors at the control module that set the communication DTC, the control module that the communication DTC was set against, and any inline harness connectors between the two control modules. Do not replace a control module based only on fretting corrosion. Refer to bulletin 09-06-03-004 for assistance with the diagnosis and repair of this condition.
8. This diagnostic can be used for any control module that should communicate with high or low speed GMLAN serial data providing the vehicle is equipped with the option that uses that control module.

Note. Terminal and connector identifiers will be different depending on the individual control module being tested. Use Schematic Reference and Connector End View Reference to identify control module's connector, B+, ignition, ground, communication enable and serial data circuit terminals.

1. Ignition OFF, disconnect the harness connector at the control module that is not communicating.
2. Ignition OFF, test for less than 10 ohms between each ground circuit terminal and ground. If greater than specified range, test the ground circuit for an open/high resistance.
3. If equipped, ignition ON, verify that a test lamp illuminates between each B+ circuit terminal and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the control circuits of the control module for a short to ground. If the circuits test normal, replace the appropriate control module.
4. If equipped, ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the control circuits of the control module for a short to ground. If the circuits test normal, replace the appropriate control module.
5. If equipped, ignition ON, verify that a test lamp illuminates between the communication enable circuit terminal OR the accessory wakeup circuit terminal and ground. If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K9 body control module, for an open/high resistance or short to ground. If the circuits test normal, replace the K9 body control module.
6. Ignition OFF, disconnect the harness connectors at the control module setting the DTC.
7. Ignition OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters away from vehicle. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 2 ohms in each GMLAN serial data circuit between the non-communicating control module and the control module setting the DTC. If greater than specified range, test the GMLAN serial data circuit for open/high resistance.
8. If all circuits test normal, replace the control module that is not communicating.

Visual/Physical Inspection

1. Inspect for aftermarket devices which could affect the operation of the systems. Refer to «Checking Aftermarket Accessories»(ref-491170-S38135034022012073000000) .
2. Inspect the easily accessible or visible system components for obvious damage or conditions which could cause the symptom.

Symptom List

Refer to a symptom diagnostic procedure from the following list in order to diagnose the symptom

1. «Scan Tool Does Not Power Up»(ref-491161-S16081984632012073000000)
2. «Scan Tool Does Not Communicate with High Speed GMLAN Device»(ref-491161-S24310616442012073000000)
3. «Scan Tool Does Not Communicate with Low Speed GMLAN Device»(ref-491161-S26637287582012073000000)
4. «Scan Tool Does Not Communicate with Chassis High Speed GMLAN Device»(ref-491161-S23994415682012073000000)

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. The scan tool will power up with the ignition OFF. Some modules however, will not communicate unless the ignition is ON and the power mode master module sends the appropriate power mode message.
2. If the B+ circuit, ground circuits, and connections of the DLC are functioning properly, the malfunction must be due to the scan tool.

1. Test for less than 2.0 ohms between the ground circuits terminal 4 of the DLC and ground, and terminal 5 of the DLC and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
2. Ignition ON, verify a test lamp illuminates between the B+ circuit terminal 16 at the DLC and ground. If the test lamp does not illuminate, test the voltage supply circuit for a short to ground or an open/high resistance.
3. If all circuits test normal, refer to the scan tool user guide.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. 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 control module on the data communication circuit was disconnected while the communication circuit is awake. Power to one or more modules was interrupted during diagnosis A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. Battery power was restored to the vehicle and control modules 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.
2. Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.
3. Communication may be available between the control module and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the control module using multiple GMLAN busses.
4. An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
5. Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC).
6. The engine may not start when there is a total malfunction of the chassis high speed GMLAN serial data bus.

Note. Each control module may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following: Control modules the vehicle is equipped with Chassis high speed GMLAN serial data circuit terminating resistors Control module locations on the chassis high speed GMLAN serial data circuits Each control module chassis high speed GMLAN serial data circuit terminals

1. Attempt to communicate with all control modules on the high speed GMLAN serial data circuit. Refer to «Data Link References»(ref-491161-S20509971852012073000000) . Communications should not be available with two or more control modules on the chassis high speed GMLAN serial data circuit. If only one control module is not communicating, diagnose that control module only. Refer to «DTC U0100-U02FF»(ref-491161-S42425685932012073000000) .
2. Disconnect the scan tool from the X84 data link connector (DLC). The following tests will be done at the DLC connector.
3. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 10 ohms between the ground circuit terminal 5 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
4. Ignition ON, test for less than 4.5 V between the serial data circuits listed below and ground: Terminal 12 Terminal 13 If greater than the specified range, test the serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuits for a Short to Voltage below.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 100 ohms between the serial data circuits listed below and ground: Terminal 12 Terminal 13 If not the specified value, test the serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to ground below.
6. Test for 50-70 ohms between the serial data circuits terminal 12 and terminal 13. If less than 35 ohms, test for a short between the serial data circuits. Refer to Testing the Serial Data Circuits for a Short between the Circuits below. If 35-50 ohms there may be a third terminating resistor between the serial data circuits. This can happen if the incorrect control module is installed. Some control modules are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness. If greater than 70 ohms but less than infinite, test the serial data circuit for an open/high resistance. Refer to Testing the Serial Data Circuits for an Open/High Resistance below. If infinite, test the serial data circuits between the DLC and the first connection to the serial data circuit for an open/high resistance.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 4.5 V between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are greater than 4.5 V. If all serial data circuits are less than the specified range, replace the control module that was disconnected.
3. Ignition OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to voltage.
4. Ignition ON, test for greater than 4.5 V between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are greater than 4.5 V. If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated: A short to voltage on the serial data circuit between 2 control modules or splice packs, if equipped A short to voltage on the serial data circuit between a control module and a terminating resistor.

Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k ohms between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k ohms. If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
3. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
4. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k ohms between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k ohms. If greater than the specified range for all serial data circuits, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated: A short to ground on the serial data circuit between 2 control modules or splice packs, if equipped A short to ground on the serial data circuit between a control module and a terminating resistor.

Testing the Serial Data Circuits for a Short between the Circuits

1. Ignition OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 ohms. If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 ohms. If each pair of serial data circuits is greater than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated: Serial data circuits shorted together between 2 control modules or splice packs, if equipped Serial data circuits shorted together between a control module and a terminating resistor A shorted terminating resistor

Testing the Serial Data Circuits for an Open/High Resistance

1. Ignition OFF, disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 ohms. If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the chassis high speed GMLAN serial data circuits at another control module, in the direction of the open circuit.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 ohms. If each pair of serial data circuits is less than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated: An open/high resistance on the serial data circuit between 2 control modules or splice packs, if equipped An open/high resistance on the serial data circuit between a control module and a terminating resistor An open/high resistance terminating resistor

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. 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 control module on the data communication circuit was disconnected while the communication circuit is awake. Power to one or more modules was interrupted during diagnosis. A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. Battery power was restored to the vehicle and control modules 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.
2. Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.
3. Communication may be available between the body control module (BCM) and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
4. An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
5. The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
6. Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
7. 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

Note. Some control modules with an internal terminating resistor have a loop in the harness that connects the internal terminating resistor to the serial data circuit. When wired this way, test these loop circuits for the appropriate failure mode short to voltage, short to ground, or open/high resistance prior to replacing the control module for each of the following tests.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 4.5 V between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are greater than 4.5 V. If all serial data circuits are less than the specified range, replace the control module that was disconnected.
3. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to voltage.
4. Ignition ON, test for greater than 4.5 V between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are greater than 4.5 V. If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated: A short to voltage on the serial data circuit between 2 control modules or splice packs, if equipped. A short to voltage on the serial data circuit between a control module and a terminating resistor.

Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k ohms between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k ohms. If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
3. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
4. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k ohms between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k ohms. If greater than the specified range for all serial data circuits, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated: A short to ground on the serial data circuit between 2 control modules or splice packs, if equipped. A short to ground on the serial data circuit between a control module and a terminating resistor. A short to ground on the serial data circuit between the DLC and the first control module or splice pack.

Testing the Serial Data Circuits for a Short between the Circuits

1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 ohms. If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 ohms. If each pair of serial data circuits is greater than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated: Serial data circuits shorted together between 2 control modules or splice packs, if equipped. Serial data circuits shorted together between a control module and a terminating resistor. Serial data circuits shorted together between the DLC and the first control module or splice pack. A shorted terminating resistor.

Testing the Serial Data Circuits for an Open/High Resistance

1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 ohms. If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the open circuit.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 ohms between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 ohms. If each pair of serial data circuits is less than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated: An open/high resistance on the serial data circuit between 2 control modules or splice packs, if equipped. An open/high resistance on the serial data circuit between a control module and a terminating resistor. An open/high resistance terminating resistor.

1. Perform the «Diagnostic System Check - Vehicle»(ref-491140-S24065023222012073000000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-491140-S09841272902012073000000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-491140-S29106564382012073000000) provides an overview of each diagnostic category.

1. 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 control module on the data communication circuit was disconnected while the communication circuit is awake. Power to one or more modules was interrupted during diagnosis A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. Battery power was restored to the vehicle and control modules 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.
2. Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
3. Communication may be available between the body control module (BCM) and the scan tool with the low speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
4. An open in the low speed GMLAN serial data circuit between the splice pack and a module will only affect that specific module. This type of failure will set a loss of communication DTC for each module effected, and the other modules will still communicate.
5. An open in the data link connector (DLC) ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
6. The engine may not start when there is a total malfunction of the low speed GMLAN serial data circuit.
7. Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed LAN 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

Note. Use the schematic to identify the following: Control modules the vehicle is equipped with Control module and splice pack locations on the low speed GMLAN serial data circuit The low speed GMLAN serial data circuit terminals for each control module, component, or splice pack

1. Attempt to communicate with all control modules on the low speed GMLAN serial data circuit. Refer to «Data Link References»(ref-491161-S20509971852012073000000) . Communications should not be available with two or more control modules on the low speed GMLAN serial data circuit. If only one control module is not communicating, diagnose that control module only. Refer to «DTC U0100-U02FF»(ref-491161-S42425685932012073000000) . If one or more control modules are communicating but not all, refer to Testing the Serial Data Circuit for an Open/High Resistance below.
2. Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
3. All access doors closed, ignition OFF for 60 seconds, test for less than 10 ohms between the ground circuit terminal 5 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
4. Ignition ON, test for less than 4.5 V between the serial data circuit terminal 1 and ground. If greater than the specified range, test the serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuit for a Short to Voltage below.
5. Ignition OFF for 60 seconds, test for greater than 100 ohms between the serial data circuit terminal 1 and ground. If less than the specified range, test the serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to ground below.
6. Disconnect the harness connector at the first splice pack closest in the circuit to the DLC.
7. Test for less than 2 ohms between the DLC serial data circuit terminal 1 and the splice pack connector serial data input terminal. If greater than the specified range, test the serial data circuit for an open/high resistance. If the circuit tests normal, replace the splice pack.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the appropriate harness connectors at the following components: All low speed serial data splice packs Any module or component between that may be between the splice packs
2. Ignition ON, test for less than 4.5 V between the DLC serial data circuit terminal 1 and ground. If greater than the specified range, repair the short to voltage.
3. Test for less than 4.5 V between each serial data circuit at each component connector and ground. If greater than the specified range, perform the appropriate test listed below: Serial data circuits with one control module, test the serial data circuit for a short to voltage. If the circuit tests normal, replace the control module. Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to voltage. If the circuits test normal, replace the control module that causes the short to voltage when connected.

Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF, disconnect the appropriate harness connectors at the following components: All low speed serial data splice packs Any module or component between that may be between the splice packs
2. Test for infinite resistance between the DLC serial data circuit terminal 1 and ground. If not the specified value, repair the short to ground.
3. Ignition OFF for 60 seconds, test for greater than 100 ohms between each serial data circuit at the splice pack and ground. If less than the specified range, perform the appropriate test listed below: Serial data circuits with one control module, test the serial data circuit for a short to ground. If the circuit tests normal, replace the control module. Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to ground. If the circuits test normal, replace the control module that causes the short to ground when connected.

Testing the Serial Data Circuit for an Open/High Resistance

1. If equipped with a second splice pack, ignition ON, verify the scan tool communicates with one or more control modules connected to the second splice pack. If all control modules on the second splice pack do not communicate, test the serial data circuit between the first and second splice packs for an open/high resistance. If the circuit tests normal, test the second splice pack for an open/high resistance. If the splice pack tests normal, replace any module or component that may be between those two splice packs.
2. If equipped with a third splice pack, ignition ON, verify the scan tool communicates with one or more control modules connected to the third splice pack. If all control modules on the third splice pack do not communicate, test the serial data circuit between the second and third splice packs for an open/high resistance. If the circuit tests normal, test the third splice pack for an open/high resistance. If the splice pack tests normal, replace any module or component that may be between the those two splice packs.
3. Ignition OFF, disconnect the splice pack containing the serial data circuits to the modules that are not communicating.
4. Install a 3 A fused jumper wire between the splice pack connector serial data input terminal and a low speed GMLAN serial data circuit that is not communicating
5. Ignition ON, verify the scan tool communicates with the control modules connected to the low speed GMLAN serial data circuit. If any control module does not communicate, test each section of the serial data circuit for an open/high resistance. If the circuits test normal, replace the control module that does not communicate when connected.

There are many components in a vehicle that rely on information from other sources, transmit information to other sources, or both. Serial data communication networks provide a reliable, cost effective, way for various components of the vehicle to "talk" to one another and share information.

GM uses a number of different communication buses to insure the timely and efficient exchange of information between control modules. When compared to each other, some of these buses are different in nature as far as speed, signal characteristics, and behavior. An example of this is the High Speed GMLAN and Low Speed GMLAN buses.

On the other hand, when other buses are compared to each other they have similar characteristics and simply operate in parallel. In this case they are used to group together components which have high interaction. Examples are the High Speed GMLAN, Powertrain Expansion, and Chassis Expansion buses. This allows them to communicate with each other on a bus with reduced message congestion insuring faster and the more timely exchange of information than if all vehicle control modules were on a single bus.

The majority of information that exists within a given network generally stays local; however some information will have to be shared on other networks. Control modules designated as Gateway's perform the function of transferring information between the various buses. A Gateway module is connected to at least 2 buses and will interact with each network according to its message strategy and transmission models.

GMLAN provides the capability for a receiving control module to monitor message transmissions from other control modules in order to determine if messages of interest are not being received. The primary purpose is to allow reasonable default values to be substituted for the information no longer being received. Additionally, a control module may set a Diagnostic Trouble Code to indicate that the control module it is expecting information from is no longer communicating.

High Speed GMLAN Circuit Description

A High Speed GMLAN Bus is used where data needs to be exchanged at a high enough rate to minimize the delay between the occurrence of a change in sensor value and the reception of this information by a control device using the information to adjust vehicle system performance.

The High Speed GMLAN serial data network consists of two twisted wires. One signal circuit is identified as GMLAN-High and the other signal circuit is identified as GMLAN-Low. At each end of the data bus there is a 120 ohms termination resistor between the GMLAN-High and GMLAN-Low circuits.

Data symbols (1's and 0's) are transmitted sequentially at a rate of 500 Kbit/s. The data to be transmitted over the bus is represented by the voltage difference between the GMLAN-High signal voltage and the GMLAN-Low signal voltage.

When the two wire bus is at rest the GMLAN-High and GMLAN-Low signal circuits are not being driven and this represents a logic "1". In this state both signal circuits are at the same voltage of 2.5 V. The differential voltage is approximately 0 V.

When a logic "0" is to be transmitted, the GMLAN-High signal circuit is driven higher to about 3.5 V and the GMLAN-Low circuit is driven lower to about 1.5 V. The differential voltage becomes approximately 2.0 (+/- 0.5) V.

Chassis High Speed GMLAN Circuit Description

The GMLAN Chassis Expansion Bus is basically a copy of the High Speed GMLAN Bus except that its use is reserved for chassis components. This implementation splits message congestion between two parallel buses helping to insure timely message transmission and reception. Sometimes communication is required between the Chassis Expansion Bus and the primary High Speed GMLAN Bus. This is accomplished by using the Electronic Brake Control Module (EBCM) as the Gateway module. Since the High Speed GMLAN Chassis Expansion Bus and primary High Speed GMLAN Bus operate in the same manner, the diagnostics for each are similar.

CAN Graphical Interface (CGI) Circuit Description

This bus is used by the Entertainment sub-system to transfer high-rate display graphics between the Radio and the Info Display Module and/or Radio/HVAC Control. The electrical characteristics of the CAN Graphical Interface (CGI) Bus are very similar to the High Speed GMLAN Bus. The message strategy and construction of messages are different however. Sometimes communication is required between the CAN Graphical Interface Bus and the Low Speed GMLAN Bus. This is accomplished by using the Radio SilverBox as the Gateway module. Since the CAN Graphical Interface Bus and primary High Speed GMLAN Bus have similar electrical characteristics, the diagnostics for each are similar.

In the case where the Info Display Module and Radio/HVAC Control are separate control modules the Info Display Module is responsible for passing information between the Radio and the Radio/HVAC Control. The Radio interfaces only with the Info Display Module and the Info Display Module then communicates with the Radio/HVAC Control through a Local Interconnect Network (LIN) interface.

A bus wake up signal will be generated by the Radio or by the Info Display Module when the system functionality is required. The communication function of the CAN Graphical Interface shall be enabled or disabled based on the voltage level of the Center Stack Wake. The network will stay awake as long as the circuit voltage is driven low, to less than 1.5 V. Communications are disabled with a high circuit voltage around 5.0 V.

The Radio can execute a warm reset of the Info Display Module if the Info Display Module fails to respond to the Radio's request. The Center Stack Reset is a low-asserted pull down output (less than 1.5 V) from the Radio to the Info Display Module and has the same electrical characteristics as those for the Center Stack Wake signal defined above.

Mid Speed GMLAN Circuit Description

The Mid Speed GMLAN Bus is very similar to the High Speed GMLAN Bus except that it uses a slower transmission rate of 125 Kbit/s. This bus is intended for use where the system response time demands that a large amount of data be transmitted in a relatively short amount of time, such as updating a graphics display. As such it has usually been used for infotainment applications. Sometimes communication is required between the Low Speed GMLAN Bus and the Mid Speed GMLAN Bus. This is accomplished by using the Radio (Silverbox) as the Gateway module. Since the Mid Speed GMLAN Bus and primary High Speed GMLAN Bus operate in a similar manner, the diagnostics for each are similar.

Low Speed GMLAN Circuit Description

Low Speed GMLAN Bus is used in applications where a high data rate is not required which allows for the use of less complex components. It is typically used for operator controlled functions where the response time requirements are slower than those required for dynamic vehicle control.

The Low Speed GMLAN Serial Data Network consists of a single wire, ground referenced bus with high side voltage drive. During on road vehicle operation data symbols (1's and 0's) are transmitted sequentially at the normal rate of 33.3 Kbit/s. For component programming only, a special high speed data mode of 83.3 Kbit/s may be used.

Unlike the high speed dual wire networks, the single wire low speed network does not use terminating resistors at either end of the network.

The data symbols to be transmitted over the bus are represented by different voltage signals on the bus. When the Low Speed GMLAN Bus is at rest and is not being driven, there is a low signal voltage of approximately 0.2 V. This represents a logic "1". When a logic "0" is to be transmitted, the signal voltage is driven higher to around 4.0 V or higher.

Local Interconnect Network (LIN) Circuit Description

The Local Interconnect Network (LIN) Bus consists of a single wire with a transmission rate of 10.417 Kbit/s. This bus is used to exchange information between a master control module and other smart devices which provide supporting functionality. This type of configuration does not require the capacity or speed of either a High Speed GMLAN Bus or Low Speed GMLAN Bus and is thus relatively simpler.

The data symbols (1's and 0's) to be transmitted are represented by different voltage levels on the communication bus. When the LIN Bus is at rest and is not being driven, the signal is in a high voltage state of approximately Vbatt. This represents a logic "1". When a logic "0" is to be transmitted, the signal voltage is driven low to about ground (0.0 V).

Communication Enable Circuit Description

Control modules on GMLAN high speed type networks, excluding the Mid Speed GMLAN Bus and CAN Graphical Interface Bus, enable or disable communication based on the voltage level of this circuit. When the circuit voltage is high (around 12 V), communications are enabled. When the circuit is low, communications are disabled.

The CAN Graphical Interface Bus is similar but uses different voltage levels. See description above for the CAN Graphical Interface Bus.