For 1990-2009 cars

### Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an identification number with at least one critical parameter within 5 seconds of beginning serial data communication, DTC U1000 or U1255 are set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

The class 2 serial data line on this vehicle is a star configuration. The following modules communicate on the class 2 serial data line:

• | The body control module (BCM)

• | The communication interface module (OnStar®), w/UE1

• | The digital radio receiver (DRR), w/U2K

• | The electronic brake control module (EBCM)

• | The inflatable restraint sensing and diagnostic module (SDM)

• | The instrument panel cluster (IPC)

• | The powertrain control module (PCM)

• | The radio

• | The transfer case shift control module (TCSCM), w/4WD

### DTC Descriptors

This diagnostic procedure supports the following DTC:

DTC U1000 Class 2 Data Link

### Conditions for Running the DTC

• | Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.

• | DTCs U1300, U1301 or U1305 do not have a current status.

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

### Conditions for Setting the DTC

A node alive message has not been received from an unidentified module within the last 5 seconds after establishing class 2 serial data communication.

### 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

When a malfunction occurs while modules are communicating, a lost communication DTC is set as a current DTC. When the modules stop communicating the current lost communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules so U1000 or U1255 are set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 are set.

### Test Description

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

1. A DTC U1001-U1254 Lost Communications with XXX with a history status may indicate the cause of U1000 or U1255.

2. The modules not communicating are the likely cause of U1000 or U1255. The modules that are available on the class 2 serial data circuit are listed in the Circuit Description.

3. The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set DTC U1001 - U1254 Lost Communications with XXX for those modules that it was monitoring.

4. The modules which can communicate indicate the module which cannot communicate. You must clear the serial data communication DTCs from these modules to avoid future misdiagnosis.

5. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.

6. The module which set U1000 or U1255 is the likely cause of the malfunction.

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 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Record the DTCs set in by each module. If using a Tech 2, use the class 2 DTC Check feature to determine which modules have DTCs set.

Did you record any DTCs in the range of U1001 - U1254 with a history status?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3 |

1. Turn ON the ignition, with the engine OFF.
2. Attempt to communicate with each module on the class 2 serial data circuit. If using a Tech 2, obtain this information using the class 2 Message Monitor feature.
3. Record all of the modules communicating on the class 2 serial data circuit.
4. Compare the list of modules which are communicating to the list given in Circuit Description.

Does any module on the class 2 serial data circuit not communicate?

Go to Step 4

Go to Step 13

4 | Test the following circuits of the module that is not communicating for an open or short to ground:

• | The battery positive voltage supply circuits

• | The switched battery positive supply circuits

• | The battery positive voltage output circuits

• | The ignition voltage input circuits

• | The ignition voltage output circuits

Refer to the following:

• | Control Module References for the applicable schematic

• | Circuit Testing

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 9

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Test the ground circuits of the module that is not communicating for an open. Refer to the following:

• | Control Module References for the applicable schematic

• | Circuit Testing

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 9

Go to Step 6

6 | Important: Inspect all connectors in the serial data communications circuit. Refer to the following:

• | Testing for Intermittent Conditions and Poor Connections

• | Connector Repairs

1. Turn OFF the ignition.
2. Test the class 2 serial data circuits of the module that is not communicating for an open. Refer to the following:

• | Circuit Testing

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 9

Go to Step 7

7 | Inspect the harness connector of the module that is not communicating for poor connections and terminal tension at the following circuits:

• | The battery positive voltage input circuits

• | The switched battery positive voltage supply

• | The battery positive voltage output circuits

• | The ignition voltage input circuits

• | The ignition voltage output circuits

• | The ground circuits

• | The class 2 serial data circuits

Refer to the following:

• | Control Module References for the applicable schematic

• | Testing for Intermittent Conditions and Poor Connections

• | Connector Repairs

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8 | Replace the module that is not communicating. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 11 | --
9 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Select the Display DTCs function for the module which was not communicating.

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

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 10

10 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 11 | --
11 | Select the Display DTCs function for the modules which had U1000 or U1255 set as a current DTC. Does the scan tool display DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 12
12 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | --
13 | Did you record any other DTCs for the modules which had U1000 or U1255 set as a current DTC? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 14
14 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Use the scan tool in order to clear the DTCs.
4. Turn OFF the ignition for at least 5 seconds.
5. Turn ON the ignition with the engine OFF.
6. Select the Display DTCs function.

Does the scan tool display U1000 set as a current DTC?

Go to Step 15

Go to Diagnostic Aids

15 | Replace the module which had U1000 or U1255 set as a current DTC. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | System OK | --