For 1990-2009 cars

### Circuit Description

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. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module. When a message that indicates the availability of the transmitter module is not received, the receiver module sets a DTC 21xx where xx is equal to the 2-digit identification number of the transmitter module.

The DTC descriptors listed below provide a method for determining which module is not communicating. A module with a GMLAN serial data circuit malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. The modules that can communicate will set a DTC indicating the module that cannot communicate.

### DTC Descriptors

This diagnostic procedure supports the following DTCs:

• | DTC U2105 Lost Communications With Engine Control System

• | DTC U2106 Lost Communications With Transmission Control System

• | DTC U2107 Lost Communications With Body Control System

• | DTC U2108 Lost Communications With ABS/TCS Control System

• | DTC U2109 Lost Communications With Powertrain Multiple Control System

• | DTC U2111 Lost Communications With Steering Control System

• | DTC U2113 Lost Communications With Sensing Diagnostic Module (SDM)

• | DTC U2116 Lost Communications With Instrument Panel Cluster (IPC)

• | DTC U2122 Lost Communications With Radio

• | DTC U2125 Lost Communication With Mobile Communications Control Systems

• | DTC U2127 Lost Communications With Front HVAC

• | DTC U2137 Lost Communications With Vehicle Theft Deterrent (VTD)

• | DTC U2171 Lost Communications With DVD Player

• | DTC U2172 Lost Communications With Digital Radio Receiver

When more than one Loss of Communication DTC is set in either one module or in several modules, diagnose the DTCs in the following order:

1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
2. The DTC which is reported the most times.

This vehicle has DTCs which include DTC Symptoms. For more information on DTC Symptoms, refer to DTC Symptom Description .

DTC Symptom | DTC Symptom Descriptor
--- | ---
00 | No Additional Information

### Conditions for Running the DTCs

• | Voltage supplied to the modules is in the normal operating voltage range.

• | The vehicle power mode requires serial data communication to occur.

• | DTC U2100 does not have a current status.

### Conditions for Setting the DTC

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

### Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

### Diagnostic Aids

• | A poor connection at the inoperative module may cause this code to set.

• | An improperly powered module may cause this code to set.

### Test Description

The numbers below refer to the step numbers on the diagnostic table:

1. Non ABS equipped vehicles may set a DTC U2108 falsely. If this is the case there is no malfunction with this system.

2. If the body control module (BCM) will not power up, the vehicle cannot establish the correct power mode. This step test for the correct power supply conditions for the BCM.

3. If DTC U2105 to U2177 are set as history and you can communicate with the suspect module, the malfunction is an intermittent communication buss, power supply, ground or internal module connection.

4. This step tests for the correct power supply conditions of the suspect module or modules.

5. This step identifies which of the GMLAN busses the module or modules use for communication.

6. The module which was not communicating due to an open in the GMLAN serial data circuits may have set Loss of Communication DTCs for those modules that it was monitoring.

7. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Is U2108 the only DTC set between the range of U2105 through U2199? | Go to Step 3 | Go to Step 4
3 | Is the vehicle equipped with ABS? | Go to Step 4 | Go to Step 14
4 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Attempt to communicate with the body control module (BCM).

Were you able to communicate with the BCM?

Go to Step 6

Go to Step 5

5 |

1. Turn the ignition OFF.
2. Disconnect the harness connectors from the BCM.
3. Test the following circuits of the BCM harness connector for the indicated condition. Refer to Circuit Testing and Wiring Repairs .

• | Switched and constant B+ supply circuits, for power

• | Ground circuits, for continuity

• | All communications circuits, for continuity

Did you find and correct the condition?

Go to Step 11

Go to Step 10

6 | Attempt to communicate with all of the modules on the vehicle. Were you able to communicate with all of the modules? | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 7
7 | Test the following circuits of the modules that do not communicate:

• | Switched B+ supply circuits

• | Constant B+ supply circuits

• | Ground circuits

Did you find and correct the condition?

Go to Step 11

Go to Step 8

8 |

1. Refer to Data Link References to identify the GMLAN buss or busses that the modules use to communicate and make a note.
2. If more than one module is not communicating, use the data link connector (DLC) schematic to determine which module is closest to the DLC. Start the analysis at that module. Refer to Data Communication Schematics .
3. Disconnect the harness connectors from the non-communicating modules.
4. Test for an open circuit in the appropriate buss between the GMLAN terminals of the disconnected harness connector and the DLC. Use the note from item one for buss identification.

Did you find and correct the condition?

Go to Step 11

Go to Step 9

9 | Inspect for poor connections at the harness connector of the non-communicating module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 11 | Go to Step 10
10 | Replace the non-communicating module. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 11 | --
11 |

1. Make sure that the scan tool has been cycled OFF for at least 15 seconds in order to reset it.
2. Install the scan tool, if necessary.
3. Turn ON the ignition, with the engine OFF.
4. Retrieve DTCs from the module that was not communicating.

Does the scan tool display any DTCs which do not begin with a "U"?

Go to Diagnostic System Check - Vehicle

Go to Step 12

12 | Use the scan tool to clear the DTCs that begin with a "U". Did you complete the action? | Go to Step 13 | --
13 | Retrieve DTCs from the modules which had the Loss of Communications DTC set. Does the scan tool display any DTCs which do not begin with a "U"? | Go to Diagnostic System Check - Vehicle | Go to Step 14
14 | Important: DTC U2108 may set in a non ABS equipped vehicle. If U2108 sets in a non ABS equipped vehicle the system is OK.

1. Attempt to communicate with all the modules on the vehicle.
2. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.

Does DTC U2105-U2177 set in any module?

Go to Step 4

System OK