Circuit Description
Modules connected to the Class 2 data link 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 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 B1300 and B1301 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
| • | Voltage supplied to the modules are in the normal operating voltage range (approximately 9-16 volts). |
| • | 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 DTC
| • | A current 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 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 Communication DTC is set current. When the modules stop communicating (ignition is turned off) the current Loss of 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 U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1255 is set.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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A Loss of Communication DTC with a history status may indicate the cause of U1255.
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The module(s) which is not communicating is the likely cause of U1255.
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The module which was not communicating due to poor connection to the Class 2 serial data circuit may have set Loss of Communication DTCs 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 U1255 may have done so due to some other condition.
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In some instances when a current U1255 is set in the radio and cannot be cleared using the scan tool, fuse V1 needs to be removed in order to clear this DTC.
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The module which set U1255 is the likely cause of the malfunction.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Did you record any DTCs in the range of U1001-U1199 with a history status? | -- | Go to Diagnostic Trouble Code (DTC) List | ||
Does any module(s) on the Class 2 serial data circuit not communicate? | -- | |||
3 | Test the battery power circuit of the module that is not communicating for an open or a short to ground. Refer to Measuring Voltage and Testing for Short to Ground in Wiring Systems. Did you find and correct the condition? | -- | ||
4 |
Did you find and correct the condition? | -- | ||
5 | Inspect for poor connections at the battery power circuit, the ground circuit, and the Class 2 serial data circuit of the module that is not communicating. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems. Did you find and correct the condition? | -- | ||
6 | Replace the module that is not communicating. Refer to the appropriate repair instructions for module replacement. Did you complete the replacement? | -- | -- | |
Does the scan tool display any DTCs which do not begin with a "U"? | -- | Go to the applicable DTC table | ||
8 | Use the scan tool in order to clear the DTCs. Did you complete the action? | -- | -- | |
Select the Display DTCs function for the module(s) which had U1255 set as a current DTC. Does the scan tool display any DTCs which do not begin with a "U"? | -- | Go to the applicable DTC table | ||
10 | Use the scan tool in order to clear the DTCs. Did you complete the action? | -- | System OK | -- |
Did you record any other DTCs for the module(s) which had U1255 set as a current DTC? | -- | Go to the applicable DTC table | ||
12 |
Does the scan tool display U1255 set as a current DTC? | -- | System OK | |
13 | Is the current U1255 set in the radio? | -- | ||
Does the scan tool display U1255 set as a current DTC? | -- | System OK | ||
Replace the module which had U1255 set as a current DTC. Refer to the appropriate repair instructions for module replacement. Did you complete the replacement? | -- | System OK | -- |
