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 ECU is replaced, ensure that the CAN bus lines are normal.

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

DTC SET CONDITIONS
This diagnostic trouble code is set when the request for the decrease of output is rejected by the engine ECU.

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. M.U.T.-III CAN bus diagnostics
Use scan tool to diagnose the CAN bus lines.
Q: Is the check result normal?
YES: Go to Step 3.
NO: Repair the CAN bus lines. On completion, go to Step 2.

STEP 2. Diagnostic trouble code recheck after resetting CAN bus lines
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
Use scan tool to check that the diagnostic trouble code is set in the engine ECU.
Q: Is any DTC set?
YES: Troubleshoot the engine ECU diagnostic trouble code.
NO: Go to Step 4.

STEP 4. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121C set?
YES: Replace the ASC-ECU. Then go to Step 5.
NO: If the trouble symptom is resolved, an intermittent malfunction such as poorly engaged connector(s) or wiring harness is suspected.

STEP 5. M.U.T.-III diagnostic trouble code
Check that the diagnostic trouble code U1417 is set in ASC-ECU.
Q: Is DTC U1417 set?
YES: Troubleshoot for the diagnostic trouble code U1417.
NO: The procedure is complete.