C121A

DTC C121A: Abnormality in steering wheel sensor initialization

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.
^ When the steering wheel sensor is replaced, always carry out calibration to make ASC-ECU learn the neutral point.
^ When the hydraulic unit (integrated with ASC-ECU) is replaced, always carry out the calibration of the steering wheel sensor, the G and yaw rate sensor and brake fluid pressure sensor.

OPERATION
Steering wheel sensor stores the neutral position learned by the scan tool. When the neutral position has not been stored in the steering wheel sensor yet, the steering wheel sensor outputs the signal indicating that it does not have neutral position.

DTC SET CONDITIONS
This diagnostic trouble code is set when ASC-ECU detects that the steering wheel sensor has not learned the neutral position yet.

PROBABLE CAUSES
^ Steering wheel sensor malfunction
^ ASC-ECU malfunction
^ Neutral position of steering wheel sensor not learned

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

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 C121A set?
YES: Go to Step 3.
NO: The procedure is complete.

STEP 3. Steering wheel sensor calibration
Perform calibration of steering wheel sensor.
Q: Has the calibration succeeded?
YES: Go to Step 4.
NO: Replace the steering wheel sensor. Then go to Step 4.

STEP 4. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121A set?
YES: Replace the ASC-ECU, and then go to Step 5.
NO: The procedure is complete.

STEP 5. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121A set?
YES: Return to Step 1.
NO: This diagnosis is complete.