C121C

DTC C121C: Torque request signal rejection

CAUTION:
^ If there is any problem in the CAN bus lines, an incorrect diagnostic trouble code may be set. Prior to this diagnosis, diagnose the CAN bus lines.
^ Whenever the ECU is replaced, ensure that the CAN bus lines are normal.
^ When the hydraulic unit (integrated with ASC-ECU) is replaced, always perform the calibration of steering wheel sensor, G and yaw rate sensor, brake fluid pressure sensor, cut valve, and inlet valve.

CIRCUIT OPERATION
ASC-ECU sends the signal to the engine control module as necessary to decrease the engine output for ASC system operation.

DTC SET CONDITIONS
This diagnostic trouble code is set when the output decrease request is rejected by the engine control module.

PROBABLE CAUSES
^ Wrong coding
^ Engine ECU malfunction
^ ASC-ECU malfunction
^ External noise interference

DIAGNOSIS

Required Special Tools:
^ MB991958 Scan Tool (M.U.T.-III Sub Assembly)
^ MB991824: Vehicle Communication Interface (V.C.I.)
^ MB991827 M.U.T.-III USB Cable
^ MB991910 M.U.T.-III Main Harness A

STEP 1. Using scan tool MB991958, diagnose the CAN bus lines.
Using scan tool MB991958, diagnose the CAN bus lines.
Q: Is the check result normal?
YES: Go to Step 3.
NO: Repair the CAN bus lines. (Refer to Troubleshooting Can Bus Diagnostics Table.) After repairing the CAN bus line, go to Step 2.

STEP 2. DTC recheck after repairing the CAN bus line
Q: Is DTC C121C set?
YES: Go to Step 3.
NO: The procedure is complete.

STEP 3. M.U.T.-III other system diagnostic trouble code
Using scan tool MB991958, check if a diagnostic trouble code is set in the engine control module. (Refer to Troubleshooting Testing and Inspection.)
Q: Is any DTC set?
YES: Troubleshoot the diagnostic trouble code of engine control module. (Refer to Troubleshooting Testing and Inspection.)
NO: Go to Step 4.

STEP 4. Scan tool service data
Check the following service data. (Refer to Scan Tool Testing and Procedures.)
^ Item 68: Allow ESP torque request
Q: Is the check result normal?
YES: Go to Step 5.
NO: Replace the engine control module. Then go to Step 5.

STEP 5. Check whether the DTC is reset.
Q: Is DTC C121C set?
YES: Replace the hydraulic unit (incorporates in ASC-ECU). Then go to Step 6.
NO: Intermittent malfunction.

STEP 6. Check whether the DTC is reset.
Q: Is DTC C121C set?
YES: Return to Step 1.
NO: The procedure is complete.