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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 XLR 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 DTC U1000 and U1255

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 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

The following modules communicate on the class 2 serial data circuit:

    • The audio amplifier
    • The body control module (BCM)
    • The CD changer (CDX)
    • The digital radio receiver (DRR)
    • The driver door module (DDM)
    • The driver door switch (DDS)
    • The driver position module (DPM)
    • The fold top controller (FTC)
    • The head up display (HUD)
    • The HVAC control module
    • The inflatable restraint sensing and diagnostic module (SDM)
    • The instrument panel cluster (IPC)
    • The passenger door module (PDM)
    • The radio
    • The remote control door lock receiver (RCDLR)
    • The steering column lock control module (SCLCM)
    • The vehicle communication interface module (VCIM)

The BCM emulates all devices that are on high speed GMLAN serial data circuits to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of the GMLAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on high speed GMLAN serial data circuits and are emulated by BCM on class 2 serial data circuit:

    • The distance sensing cruise control module (DSCC)
    • The electronic brake control module (EBCM)
    • The electronic suspension control module (ESC)
    • The engine control module (ECM)
    • The transmission control module (TCM)

DTC Descriptors

This diagnostic procedure supports the following DTCs:

DTC U1000 or U1255 Class 2 Data Link

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

Symptom

Symptom Descriptor

00

No additional information

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 message containing a critical operating parameter has not been received within the last 5-10 seconds after establishing class 2 serial data communication.

Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

Diagnostic Aids

When a malfunction such as an open fuse to a module occurs while modules are communicating, a DTC U1001-1254 Loss of XXX Communications is set current. When the modules stop communicating the current DTC U1001-1254 Loss of XXX Communications 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 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 is 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. A non communicating GMLAN module can cause the DTC setting, besides other DTC in the range of U2105-U2175 or U0100-U0299.

    2.

  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.

  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.

  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.

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

    6.

  6. If all modules are communicating, the module which set U1000 or U1255 is the likely cause of the malfunction.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

Connector End View Reference: Master Electrical Component List in Wiring Systems

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Record the DTCs set in by each module.

Did you record any DTCs in the range of U1001-U1254, or in the range of U2105-U2175, or in the range of U0100-U0299?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

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 Vehicle DTC Information 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 in Wiring Systems
    •  Wiring Repairs in Wiring Systems

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:
    3. •  Control Module References for the applicable schematic
    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

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:

   •  Connector Repairs in Wiring Systems

  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:
    3. •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

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
    •  Connector Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8

Important: Perform the module setup procedure if required.

Replace the module that is not communicating. Refer to Control Module References for the appropriate Repair Instructions.

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. Retrieve DTCs from 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 in Vehicle DTC Information

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

Retrieve DTCs from 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 in Vehicle DTC Information

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 in Vehicle DTC Information

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. Retrieve DTCs.

Does the scan tool display U1000 or U1255 set as a current DTC?

Go to Step 15

Go to Diagnostic Aids

15

Important: Perform the module setup procedure if required.

Replace the module which had U1000 or U1255 set as a current DTC. Refer to Control Module References for the appropriate Repair Instructions.

Did you complete the replacement?

System OK

--