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 the ECU is replaced, ensure that the CAN bus lines are normal.
When the steering wheel sensor is replaced, always perform calibration to make ASC-ECU learn the neutral point. (Refer to .)
When the hydraulic unit (integrated with ASC-ECU) is replaced, after turning the ignition switch ON and OFF (vehicle information from ETACS-ECU is registered), always perform the calibration of steering wheel sensor, G and yaw rate sensor, brake fluid pressure sensor, cut valve, and inlet valve.(Refer to , , , and .)

CIRCUIT OPERATION

Steering wheel sensor stores the neutral position learned using scan tool MB991958. When the neutral position has not been learned, the steering wheel sensor outputs the signal indicating that the learning has not been performed.

DTC SET CONDITIONS

This diagnostic trouble code is set when it is detected that the steering wheel sensor has not learned the neutral position.

PROBABLE CAUSES

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

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?

	Go to Step 3.

	Repair the CAN bus lines. (Refer to GROUP 54C - Troubleshooting .) After repairing the CAN bus line, go to Step 2.

STEP 2. DTC recheck after repairing the CAN bus line

Q. Is DTC C121A set?

	Go to Step 3.

	The procedure is complete.

STEP 3. Steering wheel sensor calibration

Perform calibration of the steering wheel sensor. ( Refer to

Q. Has the calibration succeeded?

	Go to Step 4.

	Replace the steering wheel sensor.(Refer to .) Then go to Step 4.

STEP 4. Check whether the DTC is reset.

Q. Is DTC C121A set?

	Replace the hydraulic unit (incorporates in ASC-ECU).(Refer to .) Then go to Step 5.

	The procedure is complete.

STEP 5. Check whether the DTC is reset.

Q. Is DTC C121A set?

	Return to Step 1.

	The procedure is complete.