Data Communication System: Diagnosis Cadillac STS I facelift

DIAGNOSTIC CODE INDEX

DIAGNOSTIC CODE INDEX

Diagnostic Starting Point - Data Communications

Begin the system diagnosis with Diagnostic System Check - Vehicle . The Diagnostic System Check - Vehicle will provide the following information

1. The identification of the control modules which are not communicating.
2. The identification of any stored diagnostic trouble codes (DTCs) and their status.

The use of the Diagnostic System Check - Vehicle will identify the correct procedures to begin vehicle diagnosis. These must be performed before system DTC or symptom diagnosis.

Diagnostic Instructions

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

The control module microprocessor must be active/awake.

The KAM in the control module has been reset. It is a normal occurrence when battery positive voltage or ground is removed from the module, such as a battery disconnect.

The microprocessor reverts back to the base programmed critical operating data until new data is learned and stored in KAM.

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-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

The control module runs the program to detect an EEPROM checksum error after each wake-up. The only requirements are battery positive voltage and ground.

The control module retains an inverse copy of the digital value stored in certain blocks of memory in the EEPROM. The control module then reads the information from those certain blocks and adds the stored inverse value to the current value. If they do not equal 0, the module sets the DTC.

The control module reverts to base operation values programmed for those blocks of data that have failed the check sum test.

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, without a repeat of the malfunction.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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 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-U0299 or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) or «DTC U2105-U2199»(ref-338128-S17506902922009081700000) .
2. Ignition OFF, test for less then 5 ohms between the applicable module ground circuit and ground. If greater then the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect the applicable module harness connector.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable 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 then 5 ohms between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U0002. If greater then the specified range, test the circuit for and open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

Circuit Description

The GMLAN serial data circuits are controller area network (CAN) high and/or low speed serial data buses used to communicate information between the control modules. Typical data transmission speeds must be high enough to ensure that a required real-time response is maintained. There are two different types of GMLAN serial data circuits, the High speed 2-wire circuit and Low speed single wire circuit. The GMLAN serial data circuits also communicate directly to the data link connector (DLC). Messages are interpreted by the externally connected CANdi module which acts as a transceiver for the scan tool.

Modules connected to the GMLAN serial data circuits monitor communications during normal vehicle operation, where operation information is exchanged among the modules. All the modules on the GMLAN networks, maintain 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.

1. Supply voltage to the modules are in the normal operating range.
2. The vehicle is in the RUN power mode.

The module setting the DTC has attempted to establish communications on the GMLAN circuits for more than 7 times.

1. The module suspends all message transmission.
2. The module uses default values for all parameters received on the serial data circuits.
3. The module 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 module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

This DTC cannot be retrieved with a current status. Diagnosis of a current DTC is accomplished via Scan Tool Does Not Communicate with High Speed GMLAN Device . Refer to Data Link References for the correct procedure for the specific module(s) and the busses they use to communicate.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

Diagnostic Fault Information

The system voltage is between 9-16 volts.

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. 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're 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. 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-338128-S02446143102009081700000) to determine if the 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. 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. Determine the module that is not communicating. Refer to «Control Module U Code List»(ref-338128-S26972651412009081700000) .
2. Ignition OFF, disconnect the necessary harness connector of the module that is not communicating.
3. Test for less than 5 ohms between each ground circuit terminal and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
4. 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 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 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.
6. 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 circuit for an open/high resistance. If the circuits test normal, replace the BCM.
7. Ignition OFF for 60 seconds, test for less than 5 ohms between the serial data circuit terminals and the 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 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.
8. If all circuits test normal, replace the control module that is not communicating.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

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

A supervised periodic message that includes the transmitter module availability is missing or invalid.

1. The EBCM disables the VSES for the duration of the ignition cycle.
2. The driver information center (DIC) displays the STABILITRAK Off message.
3. The ABS/TCS remains functional.

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 SCM.
3. Possible causes of this DTC are as follows: Inspect the mounting of the Yaw sensor to ensure proper orientation and installation of the sensor. On a level surface, read the Yaw sensor offset value to ensure a value within the proper range.

1. Ignition OFF, disconnect the harness connector at the yaw rate/lateral acceleration sensor.
2. Ignition OFF for 60 seconds, test for less than 5 ohms between the low reference circuit terminal 1 and ground. If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the yaw rate/lateral acceleration sensor.
3. Ignition ON, test for ignition voltage between the ignition voltage circuit terminal 4 and ground. If not within the specified range, test the ignition voltage circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the yaw rate/lateral acceleration sensor.
4. Ignition OFF, disconnect the harness connector X3 at the SCM, and X3 at the EBCM.
5. Ignition ON, test for less than 1 volt between the following terminal listed below and ground. SCM serial data circuit terminal 52 SCM serial data circuit terminal 72 If greater than the specified value, test the appropriate serial data circuit for a short to voltage. If the circuit tests normal, replace the yaw rate/lateral acceleration sensor.
6. Ignition OFF, test for infinite resistance between the following terminal listed below and ground. SCM serial data circuit terminal 52 SCM serial data circuit terminal 72 If not the specified value, test the appropriate serial data circuit for a short to ground. If the circuit tests normal, test or replace the yaw rate/lateral acceleration sensor
7. Test for less than 2 ohms between the appropriate serial data circuit terminals listed below. Serial data circuit terminal 52 at the SCM harness connector and terminal 2 at the yaw rate/lateral acceleration sensor harness connector. Serial data circuit terminal 72 at the SCM harness connector and terminal 3 at the yaw rate/lateral acceleration sensor harness connector. If greater than the specified range, test the appropriate serial data circuit for an open/high resistance. If the circuit tests normal, replace the yaw rate/lateral acceleration sensor.
8. Test for infinite resistance between the SCM serial data circuit terminal 52 and the SCM serial data circuit terminal 72. If not the specified value, repair the serial data circuits for a short together.
9. If all circuits test normal, replace the SCM.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

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

A supervised periodic message that includes the transmitter module availability is missing or invalid.

1. The EBCM disables the VSES for the duration of the ignition cycle.
2. The driver information center (DIC) displays the STABILITRAK Off message.
3. The ABS/TCS remains functional.

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. The car should not pull in either direction causing the steering wheel to be OFF center while driving straight on a level surface.
2. Use the DTC Descriptor list above to determine the module which is not communicating.
3. If there are multiple non-communicating modules choose the one closest to the EBCM.

1. Ignition OFF, disconnect the harness connector at the steering angle sensor.
2. Ignition OFF for 60 seconds, test for less than 5 ohms between the low reference circuit terminal 6 and ground. If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the SAS.
3. Ignition ON, test for ignition voltage between the ignition voltage circuit terminal 5 and ground. If not within the specified range, test the ignition voltage circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the SAS.
4. Ignition OFF, disconnect the harness connector X3 at the SCM, and X3 at the EBCM.
5. Ignition ON, test for less than 1 volt between the following terminal listed below and ground. SCM serial data circuit terminal 52 SCM serial data circuit terminal 72 If greater than the specified value, test the appropriate serial data circuit for a short to voltage. If the circuit tests normal, test or replace the steering angle sensor.
6. Ignition OFF, test for infinite resistance between the following terminal listed below and ground. SCM serial data circuit terminal 52 SCM serial data circuit terminal 72 If not the specified value, test the appropriate serial data circuit for a short to ground. If the circuit tests normal, test or replace the steering angle sensor.
7. Test for less than 2 ohms between the appropriate serial data circuit terminals listed below. Serial data circuit terminal 52 at the SCM harness connector and terminal 1 at the steering angle sensor harness connector. Serial data circuit terminal 72 at the SCM harness connector and terminal 3 at the steering angle sensor harness connector. If greater than the specified range, test the appropriate serial data circuit for an open/high resistance. If the circuit tests normal, replace the steering angle sensor.
8. Test for infinite resistance between the SCM serial data circuit terminal 52 and the SCM serial data circuit terminal 72. If not the specified value, repair the serial data circuits for a short together.
9. If all circuits test normal, replace the SCM.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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. DTC C0800 03 does not have a current status.

Supervised periodic messages from the IPM have not been received within a predetermined period of time.

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-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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.

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. With a scan tool, verify DTC U0100-U0299 or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) or «DTC U2105-U2199»(ref-338128-S17506902922009081700000)
2. Ignition OFF, test for less then 5 Ohms of resistance between the applicable modules ground circuit and ground. If greater then the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect the applicable modules harness connector.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable 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 then 5 Ohms of resistance between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U1820. If greater then the specified range, test the circuit for and open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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.

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. With a scan tool, verify DTC U0100-U0299 or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) or «DTC U2105-U2199»(ref-338128-S17506902922009081700000)
2. Ignition OFF, test for less then 5 Ohms of resistance between the applicable modules ground circuit and ground. If greater then the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect the applicable modules harness connector.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable 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 then 5 Ohms of resistance between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U1822. If greater then the specified range, test the circuit for and open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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.

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. With a scan tool, verify DTC U0100-U0299 or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) or «DTC U2105-U2199»(ref-338128-S17506902922009081700000)
2. Ignition OFF, test for less then 5 Ohms of resistance between the applicable modules ground circuit and ground. If greater then the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect the applicable modules harness connector.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable 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 then 5 Ohms of resistance between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U1823. If greater then the specified range, test the circuit for and open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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.

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. With a scan tool, verify DTC U0100-U0299 or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) or «DTC U2105-U2199»(ref-338128-S17506902922009081700000)
2. Ignition OFF, test for less then 5 Ohms of resistance between the applicable modules ground circuit and ground. If greater then the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect the applicable modules harness connector.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable 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 then 5 Ohms of resistance between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U1824. If greater then the specified range, test the circuit for and open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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.

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. With a scan tool, verify DTC U0100-U0299 or U2105-U2199 is not set as current. If DTC is set, refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) or «DTC U2105-U2199»(ref-338128-S17506902922009081700000)
2. Ignition OFF, test for less then 5 Ohms of resistance between the applicable modules ground circuit and ground. If greater then the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect the applicable modules harness connector.
4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable 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 then 5 Ohms of resistance between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U1825. If greater then the specified range, test the circuit for and open/high resistance.
6. If all circuits test normal, replace the applicable control module.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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.

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

1. The vehicle stability enhancement system (VSES), if equipped, and traction control system (TCS) will not function.
2. System OFF lamps will illuminate for these systems.

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.

Yaw Rate Sensor DTC U1826

1. Use the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector at the Yaw Rate Sensor.
3. Ignition ON, verify that a test lamp illuminates between the Yaw Rate Sensor voltage (12-volt ref) circuit terminal 4 and ground. If the test lamp does not illuminate, test the circuit for a short to ground, or open/high resistance. If the circuit tests normal, replace the yaw/accelerometer sensor.
4. Ignition OFF, test for less than 5 ohms between the Yaw Rate Sensor low reference circuit terminal 1 and ground. If greater than the specified value, test the ground circuit for an open/high resistance. If the circuit tests normal, replace the yaw/accelerometer sensor.
5. Test for less than 1 ohm in the CAN bus serial data circuits between the Yaw Rate Sensor and the SCM. If greater than 1 ohm, test the serial data circuits terminals 2 and 3 for open/high resistance.
6. If all circuits test normal, replace the SCM.

Steering Wheel Position Sensor (Steering Angle Sensor) DTC U1827

1. Use the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector at the Steering Wheel Position Sensor.
3. Ignition ON, verify that a test lamp illuminates between the Steering Wheel Position Sensor voltage (12-volt ref) circuit terminal 5 and ground. If the test lamp does not illuminate, test the circuit for a short to ground, or open/high resistance. If the circuit tests normal, replace the Steering Wheel Position Sensor.
4. Ignition OFF, test for less than 5 ohms between the Steering Wheel Position Sensor low reference circuit terminal 6 and ground. If greater than the specified value, test the ground circuit for an open/high resistance. If the circuit tests normal, replace the Steering angle Position Sensor.
5. Test for less than 1 ohm in the CAN bus serial data circuits between the Steering Wheel Position Sensor and the Yaw Rate Sensor. If greater than 1 ohm, test the serial data circuits terminals 3 and 1 for open/high resistance.
6. If all circuits test normal, replace the SCM.
7. Test for less than 1 ohm in the CAN bus serial data circuits between the Steering Wheel Position Sensor and the Active Front/Power Steering Control Module. If greater than 1 ohm, test the serial data circuits terminals 4 and 2 for open/high resistance.
8. If all circuits test normal, replace the steering angle position sensor.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

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

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. Using the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector of the module that is not communicating.
3. Ignition ON, verify that a test lamp illuminates between the high speed GMLAN serial data wake up circuit OR the accessory wake up serial data circuit and ground. If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuits test normal, replace the BCM.
4. Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground. If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module.
5. Verify that a test lamp illuminates between all ignition voltage circuits and ground. If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module.
6. Test for less than 1 ohm of resistance between the module ground circuits and ground. If greater than 1 ohm, test the ground circuit for an open.
7. Test for less than 1 ohm of resistance in the GMLAN serial data circuit(s) between the non-communicating module and the module that displays the DTC. If greater than 1 ohm of resistance, test the GMLAN serial data circuit(s) for open/high resistance.
8. If all circuits test normal, replace the module that is not communicating.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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, «DTC U0100-U0299»(ref-338128-S27857501412009081700000) .
2. An intermittent condition is likely to be caused by a short on the GMLAN serial data circuits. Use the «DTC U0100-U0299»(ref-338128-S27857501412009081700000) procedure in order to isolate an intermittent condition.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using the diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

1. The system voltage is between 9-16 volts.
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 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 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 data link connector (DLC).
3. Use «Data Link References»(ref-338128-S02446143102009081700000) to determine if the module uses high or low speed GMLAN serial data communications
4. Some 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 module output voltage circuit is shorted to ground before replacing the module.
5. This diagnostic can be used for any module that should communicate with high or low speed GMLAN serial data providing the vehicle is equipped with the option that uses that module.

1. Using the DTC Descriptor and Diagnostic Aids, determine the module that is not communicating that should be tested.
2. Ignition OFF, disconnect the harness connector of the module that is not communicating.
3. Test for less than 2 ohms between the module ground circuits and ground. If greater than specified range, test the ground circuit for an open/high resistance.
4. Ignition ON, verify that a test lamp illuminates between the high speed GMLAN serial data wake up circuit OR the accessory wake up serial data circuit and ground. If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuits test normal, replace the BCM.
5. Ignition ON, verify that a test lamp illuminates between all B+ circuits 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 positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the appropriate module.
6. Verify that a test lamp illuminates between all ignition voltage circuits and ground. If the test lamp does not illuminate, test the ignition voltage circuits for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the appropriate module.
7. Test for less than 1 ohm in the GMLAN serial data circuits between the non-communicating module and the module that displays the DTC. If greater than specified range, test the GMLAN serial data circuits for open/high resistance.
8. If all circuits test normal, replace the 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-338151-S01119782592009081700000) .
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-338128-S02890222942009081700000)
2. «Scan Tool Does Not Communicate with High Speed GMLAN Device»(ref-338128-S02052815682009081700000)
3. «Scan Tool Does Not Communicate with Low Speed GMLAN Device»(ref-338128-S10340599422009081700000)

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) provides an overview of each diagnostic category.

1. Perform the «Diagnostic System Check - Vehicle»(ref-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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're 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 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. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed.
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. The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus. 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. Tire Pressure Monitor (TPM) system warning lights.

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 High speed GMLAN serial data circuit terminating resistors Control module locations on the high speed GMLAN serial data circuits Each control module's 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-338128-S02446143102009081700000) . Communications should not be available with two or more control modules on the high speed GMLAN serial data circuit. If only one control module is not communicating, diagnose that control module only; refer to «DTC U0100-U0299»(ref-338128-S27857501412009081700000) .
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 circuits listed below and ground: Terminal 6 Terminal 14 If greater than the specified range, test the serial data circuit for a short to voltage; refer to «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuits for a Short to Voltage.
5. Ignition OFF for 60 seconds, test for greater than 100 ohms between the serial data circuits listed below and ground: Terminal 6 Terminal 14 If not the specified value, test the serial data circuit for a short to ground; refer to «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuits for a Short to ground.
6. Test for 50-70 ohms between the serial data circuits terminal 6 and terminal 14. If less than 35 ohms, test for a short between the serial data circuits; refer to «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuits for a Short between the Circuits. 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 «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuits for an Open/High Resistance. 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 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 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 for 60 seconds, 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 for 60 seconds, 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 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 high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF for 60 seconds, test for less than 35 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 35 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 for 60 seconds, test for less than 35 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 35 ohms. If each pair of serial data circuits is within 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 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 for 60 seconds, test for greater than 100k 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 100k 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 for 60 seconds, test for greater than 100k 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 100k 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 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-338149-S41903538652009081700000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-338149-S37154416862009081700000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-338149-S02595537362009081700000) 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're 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 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 DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a No CANdi Module Detected message will be displayed.
6. An open between the DLC and Instrument Panel Module (IPM) will only effect the communication with the scan tool. The vehicle modules will still communicate, and the vehicle will start.
7. The engine will not start when there is a total malfunction of the low speed GMLAN serial data circuit.
8. 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. Tire Pressure Monitor (TPM) system warning light.

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

1. Attempt to communicate with all control modules on the low speed GMLAN serial data circuit, refer to «Data Link References»(ref-338128-S02446143102009081700000) . 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-U0299»(ref-338128-S27857501412009081700000) . If one or more control modules are communicating but not all, refer to «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuit for an Open/High Resistance.
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 «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuit for a Short to Voltage.
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 «Circuit/System Testing»(ref-338128-S37045403692009081700000) , under Testing the Serial Data Circuits for a Short to ground.
6. Disconnect the X3 harness connector at the Instrument Panel Module (IPM).
7. Test for less than 2 ohms between the DLC serial data circuit terminal 1 and IPM connector X3 terminal 22. If greater than the specified range, test the serial data circuit for an open/high resistance. If the circuits test normal, replace the IPM.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the appropriate harness connectors at the following components: IPM Inflatable Restraint Sensing and Diagnostic Module (SDM) Splice pack JX 201 Splice pack JX 300 Splice pack JX 303
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 the each component connector and ground. If greater than the specified range, perform the appropriate test listed below

1. 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.
2. 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: IPM Inflatable Restraint Sensing and Diagnostic Module (SDM) Splice pack JX 201 Splice pack JX 300 Splice pack JX 303
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

1. 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.
2. 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 ground when connected.

Testing the Serial Data Circuit for an Open/High Resistance

1. If equipped with a JX 303 splice pack, ignition ON, verify the scan tool communicates with one or more control modules connected to the JX 303 splice pack. If all control modules on the JX 303 splice pack do not communicate, test the serial data circuit between the JX 303 splice pack and the Inflatable Restraint Sensing and Diagnostic Module (SDM) for an open/high resistance. If the circuit tests normal, test the splice pack for an open/high resistance. If the splice pack tests normal, replace the SDM.
2. If equipped with a JX 300 splice pack, ignition ON, verify the scan tool communicates with one or more control modules connected to the JX 300 splice pack. If all control modules on the JX 300 splice pack do not communicate, test the serial data circuit between the JX 201 and JX 300 splice packs for an open/high resistance. If the circuit tests normal, test the JX 201 splice pack for an open/high resistance.
3. Ignition OFF, disconnect one of the harness connectors listed below: JX 201 if the control modules not communicating are connected to the JX 201 splice pack JX 300 if the control modules not communicating are connected to the JX 300 splice pack JX 303 if the control modules not communicating are connected to the JX 303 splice pack
4. Install a 3A fused jumper wire between the appropriate disconnected splice pack terminal listed below and a low speed GMLAN serial data circuit that is not communicating: JX 201 terminal 13 JX 300 terminal 6 JX 303 terminal 15
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.

The communication among modules is performed through the low speed and high speed GMLAN serial data circuits. The modules that need real time communication are attached to the high speed GMLAN network. The instrument panel module (IPM) is the gateway between the networks. The purpose of the gateway is to transfer information from one network to another. The gateway will interact with each network according to that network's transmission model. Refer to Body Control System Description and Operation for more information about the gateway.

GMLAN network management supports selective start up and is based on virtual networks. A virtual network is a collection of signals started in response to a vehicle event. The starting of a virtual network signifies that a particular aspect of the vehicles's functionality has been requested. A virtual network is supported by virtual devices which represents a collection of signals owned by a single physical device. So, any physical device can have one or more virtual devices. The signal supervision is the process of determining whether an expected signal is being received or not. Failsofting is the ability to substitute a signal with a default value or a default algorithm, in the absence of a valid signal. Some messages are also interpreted as a "heartbeat" of a virtual device. If such a signal is lost, the application will set a no communication code against the respective virtual device. This code is mapped on the Tech 2 screen as a code against the physical device. Note that a loss of serial data DTC does not normally represent a failure of the module that set it.

High Speed GMLAN Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit. The serial data is transmitted on 2 twisted wires that allow speed up to 500 Kbps. The twisted pair is terminated with two 120 ohms resistors, one is internal to the engine control module (ECM) and the other is internal to the instrument panel module (IPM). The resistors are used to reduce noise on the high speed GMLAN bus during normal vehicle operation. The high speed GMLAN is a differential bus. The high speed GMLAN serial data bus (+) and high speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level. The idle level which is approximately 2.5 volts is considered a recessive transmitted data and is interpreted as a logic 1. Driving the lines to their extremes adds 1 volt to high speed GMLAN serial data bus (+) and subtracts 1 volt from high speed GMLAN serial data bus (-) wire. This dominant state is interpreted as a logic 0.

Two different outputs are used by IPM to activate the high speed GMLAN bus

1. Accessory wake up, active in all power modes when IPM is awake except Crank and in a remote start request
2. High speed communication enable, active in all power modes when IPM is awake and in a remote start request

The accessory wake up signal wakes up the ECM and the transmission control module (TCM) only. This output is disabled in Crank power mode for internal fail enable of the ECM and TCM. The high speed communication enable signal wakes up all the other modules on the high speed GMLAN bus.

Low Speed GMLAN Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the low speed GMLAN serial data circuit. The serial data is transmitted over a single wire to the appropriate control modules. The transmission speed for GMLAN low speed is up to 83.33 Kbps. This high speed is used for programming purposes only. Under normal vehicle operating conditions the speed of the bus is 33.33 Kbps. This protocol produces a simple pulse train sent out over the low speed GMLAN serial data bus. To wake up the control modules connected to the low speed GMLAN serial data bus, a wake-up pulse of 10 volts is sent out over the bus.