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 (Node Alive messages). 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 about five seconds of beginning serial data communication, DTC U1000 or DTC 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.
Conditions for Running the DTC
| • | Diagnostic trouble codes B1327, U1300, U1301, and U1305 do not have a current status. Any particular device on the class 2 serial data circuit will support some subset of these DTCs. |
| • | The vehicle power mode (ignition switch position) requires serial data communication to occur. |
Conditions for Setting the DTC
At least one critical operating parameter has not been associated with an identification number within about five seconds after beginning serial data communication.
Action Taken When the DTC Sets
The module uses a default value for the missing parameter.
Conditions for Clearing the MIL/DTC
| • | A current U1000 or U1255 will clear when all critical operating parameters for the module have been associated with an identification number or at the end of the current ignition cycle. |
| • | A history U1000 or U1255 will clear upon receipt of a scan tool Clear DTCs command. |
Diagnostic Aids
When a malfunction (such as a blown fuse to a module) occurs while modules are communicating, a Loss of XXX Communication DTC is set current. When the modules stop communicating (ignition is turned off) the current Loss of XXX Communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the blown 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.
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A Loss of Communication DTC U1001 - U1199 with a history status may indicate the cause of U1000 or U1255.
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The modules not communicating is the likely cause of U1000 or U1255. The modules available are listed under Class 2 Serial Data Link. Refer to Data Link Communications Circuit Description .
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The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set Loss of Communication DTCs U1001 - U1199 for those modules that it was monitoring.
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The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
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If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
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The module which set U1000 or U1255 is the likely cause of the malfunction.
Step | Action | Yes | No |
|---|---|---|---|
1 | Did you record any DTCs in the range of U1001-U1199 with a history status? | Go to Step 2 | |
2 |
Does any modules on the class 2 serial data circuit not communicate? | Go to Step 3 | Go to Step 12 |
3 | Test the battery positive voltage and/or ignition voltage circuits of the module that is not communicating for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 10 | Go to Step 4 |
4 |
Did you find and correct the condition? | Go to Step 10 | Go to Step 5 |
5 |
Did you find and correct the condition? | Go to Step 10 | Go to Step 6 |
6 | Inspect for poor connections at the battery positive voltage and/or ignition voltage circuits, the ground circuits, and the class 2 serial data circuits harness connectors of the module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 8 | Go to Step 7 |
7 | Replace the module which is not communicating. Refer to Control Module References in Body Control Systems for the appropriate Repair Instructions for the module replacement. Did you complete the replacement? | Go to Step 10 | -- |
8 |
Does the scan tool display any DTCs which do not begin with a U? | Go to the Control Module References in Body Control Systems for applicable Diagnostic System Check | Go to Step 9 |
9 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 10 | -- |
10 | 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 the Control Module References in Body Control Systems for applicable Diagnostic System Check | Go to Step 11 |
11 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- |
12 | Did you record any other DTCs for the modules which had U1000 or U1255 set as a current DTC? | Go to the Control Module References in Body Control Systems for applicable Diagnostic System Check | Go to Step 13 |
13 |
Does the scan tool display U1000 or U1255 set as a current DTC? | Go to Step 14 | Go to Diagnostic Aids |
14 | Replace the module which had U1000 or U1255 set as a current DTC. Refer to Control Module References in Body Control Systems for the appropriate Repair Instructions for the module replacement. Did you complete the replacement? | System OK | -- |
