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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 (Refer to GROUP 54C - CAN Bus Line Diagnostic Flow ).
  • 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 (Refer to ).
  • 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 (Refer to , and ).
 
 
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
STEP 1. Using scan tool MB991958, diagnose the CAN bus lines.
Use scan tool to diagnose the CAN bus lines.
Is the check result normal?
Go to Step 3.
Repair the CAN bus lines (Refer to GROUP 54C - CAN Bus Diagnostics Table ). On completion, go to Step 2.
 
STEP 2. DTC recheck after resetting CAN bus lines
Is DTC C121A set?
Go to Step 3.
The procedure is complete.
 
STEP 3. Steering wheel sensor calibration
Perform calibration of steering wheel sensor (Refer to ).
Has the calibration succeeded?
Go to Step 4.
After the steering wheel sensor is replaced, perform the steering wheel sensor calibration to make ASC-ECU learn the neutral point again (Refer to ). Then go to Step 4.
 
STEP 4. Check whether the DTC is reset.
Erase the DTC.
Is DTC C121A set?
Replace the hydraulic unit (integrated with ASC-ECU) (Refer to ), and then go to Step 5.
The procedure is complete.
 
STEP 5. Check whether the DTC is reset.
Erase the DTC.
Is DTC C121A set?
Return to Step 1.
The procedure is complete.