U0121-Lost Communication with Antilock Brake System (ABS)

When monitored U0121 Code

• With the ignition on.
• Battery voltage between 10 and 16 volts.
• Ignition Off Draw (IOD) fuse installed.
• Totally Integrated Power Module (TIPM) or Body Control Module (BCM).

Set condition:

• Bus messages not received from the ABS for approximately two to five seconds.

Possible causes:

• CAN C Bus (+) circuit shorted to voltage.
• CAN C Bus (-) circuit shorted to ground.
• DTC related to battery voltage, ignition, or VIN missing.
• TIPM/BCM configured correctly.
• ABS power and ground.
• Antilock Brake System.
• Module that set this DTC.

Diagnose U0121-Lost Communication with Antilock Brake System (ABS)

Step 1. Verify the Diagnostic Trouble Code (DTC) is active

Make sure the IOD fuse is installed and battery voltage is between 10 and 16 volts before proceeding. With the scan tool, read active DTC.

Is this DTC active ?

Yes, go to step 2.

No, refer to Sitemap/Labels-Non dtc diagnostic (Communication) and perform the Stored Lost Communication.

Step 2. Check for any of the following active DTC

With the scan tool, read all active DTC from all modules. Check for TIPM configured, CAN C hardware electrical, VIN Missing/Mismatch, battery or ignition related DTC.

Does the scan tool display any active DTC to the conditions listed above ?

Yes, go to and perform the appropriate diagnostic.

No, go to step 3.

Step 3. Verify the ABS is active on the Bus

Turn the ignition on, with the scan tool, view all modules. Verify the ABS is active on the Bus.

Is the ABS active on the Bus ?

Yes, go to step 4.

No, refer to Sitemap/Labels-Non Dtc diagnostic (Communication) and perform the No Response from ABS.

Step 4. Check for additional communication related DTC

With the scan tool, view all modules.

Is there more than one module with active DTC ''Logged Against'' the ABS ?

Yes, replace/update the ABS Module. Perform the ABS Verification Test.
No, go to step 5.

Step 5. Clear DTC in Module setting fault
With the scan tool, select the module setting the DTC against the ABS, clear active DTC.

Is this DTC still active ?

Yes, replace/update the module that set this DTC. Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the Body Verification Test.
No, go to step 6.

Step 6. Cycle Ignition
Cycle the ignition from RUN to OFF three times ending with the key in the RUN position.

Does this DTC become active ?

Yes, go to step 7.
No, the condition is not present at this time. Using the wiring diagram/schematic as a guide, inspect the wiring for chafed, pierced, pinched, and partially broken wires and the wiring harness connectors for broken, bent, pushed out, and corroded terminals.

Step 7. Check for additional Lost Communication faults

With the scan tool, view all modules.

Does the TIPM/BCM and other CAN C Bus modules show any Lost Communication DTC with the remaining CAN C Bus modules ?

Yes, go to step 8.
No, replace/update the module that set this DTC. Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the Body Verfication Test.

Step 8. CAN C Bus (+)  circuit shorted to voltage
Measure the voltage between the CAN C Bus (+) circuit and ground.

Is the voltage above 5.0 volts ?

Yes, repair the short to voltage in the CAN C Bus (+) circuit.
No, go to step 9.

Step 9. CAN C Bus (-) circuit shorted to ground
Turn the ignition off, measure the resistance between ground and the CAN C Bus (-) circuit.

Is the resistance below 10k ohms ?

Yes, repair the short to ground in the CAN C Bus (-) circuit.
No, replace/update the module that set this DTC. Perform the module being replaced. If there is no verification test for the associated module perform the Body Verification Test.

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