Pinpoint Test H: DTC U0121

Auxiliary Brake System - Trailer Brake Control (TBC) Module

Pinpoint Tests

Pinpoint Test H: DTC U0121

Normal Operation
- DTC U0121 (Lost Communication With Anti-Lock Brake System (ABS) Control Module) - sets in the Trailer Brake Control (TBC) module if data messages such as brake torque information are not received from the ABS module. For a complete list of all network messages, refer to Module Communications Network Testing and Inspection.

This pinpoint test is intended to diagnose the following:


- ABS module
- TBC module

PINPOINT TEST H: DTC U0121

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H1 VERIFY THE CUSTOMER CONCERN

- Ignition ON.
- Verify that there is an observable symptom present.
- Is an observable symptom present?

Yes
GO to H2.

No
The system is operating normally at this time. The DTC may have been set due to high network traffic or an intermittent fault condition.

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H2 CHECK THE COMMUNICATION NETWORK

- Ignition ON.
- Enter the following diagnostic mode on the scan tool: Network Test.
- Carry out the network test.
- Does the ABS module pass the network test?

Yes
GO to H3.

No
REFER to Module Communications Network Testing and Inspection to diagnose the ABS module does not respond to the scan tool.

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H3 RETRIEVE THE RECORDED DTCs FROM THE TBC AND ABS MODULE SELF-TESTS

- Check for recorded TBC and ABS module DTCs from the self-test.
- Is DTC U3003:16 (or B1318) or DTC U3003:17 (or B1317) recorded?

Yes
For the TBC module, REFER to DTC Charts. Diagnostic Trouble Code Tests and Associated Procedures

For the ABS module, REFER to Vehicle Dynamic Systems Testing and Inspection.

No
GO to H4.

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H4 CHECK FOR DTC U0121:87 SET IN OTHER MODULES

- Clear the DTCs.
- Ignition OFF.
- Ignition ON.
- Wait 10 seconds.
- Enter the following diagnostic mode on the scan tool: Self-Test.
- Retrieve the continuous memory DTCs from all modules.
- Is DTC U0121:87 or DTC U0121 set in more than one module?

Yes
INSTALL a new ABS module. REFER to Vehicle Dynamic Systems Testing and Inspection. CLEAR the DTCs. REPEAT the ABS module self-test.

No
INSTALL a new TBC module. REFER to Trailer Brake Control (TBC) Module Service and Repair. TEST the system for normal operation.

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