Code No. C121D Abnormality in brake fluid pressure sensor circuit


caution
  • If there is any problem in the CAN bus lines, an incorrect diagnosis code may be set. Prior to this diagnosis, diagnose the CAN bus lines (Refer to GROUP 54C, CAN Bus Diagnosis Table ).
  • 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 ).
  • Whenever ECU is replaced, ensure that the CAN bus lines are normal.

OPERATION
The hydraulic sensor is incorporated in the hydraulic unit. When the brake pedal is depressed, the pressure sensor detects the brake pressure applied from the master cylinder, converts this pressure into the voltage signal, and outputs it.

DIAGNOSIS CODE SET CONDITIONS
When the pressure sensor output signal is not within the standard value range, ASC-ECU sets this diagnosis code.

PROBABLE CAUSES

  • Incorrect brake pedal height
  • Incorrect adjustment of the stop lamp switch
  • Master cylinder malfunction
  • Brake booster malfunction
  • ASC-ECU malfunction

DIAGNOSIS PROCEDURE

STEP 1. M.U.T.-III CAN bus diagnostics
Use M.U.T.-III to diagnose the CAN bus lines.

Q. Is the check result normal?
Go to Step 3.
Repair the CAN bus lines (Refer to GROUP 54C - CAN Bus Diagnosis table ). On completion, go to Step 2.

STEP 2. Diagnosis code recheck after resetting CAN bus lines

Q. Is diagnosis code No. C121D set?
Go to Step 3.
This diagnosis is complete.

STEP 3. Brake pedal check
Refer to GROUP 35A - On-vehicle Service, Brake Pedal Check and Adjustment .

Q. Is the check result normal?
Go to Step 4.
Go to GROUP 35A - On-vehicle Service, Brake Pedal Check and Adjustment . Then go to Step 7.

STEP 4. Check the installation condition of the stop lamp switch.
Refer to GROUP 35A - On-vehicle Service, Brake Pedal Check and Adjustment .

Q. Is the check result normal?
Go to Step 5.
Install the stop lamp switch correctly (Refer to GROUP 35A - On-vehicle Service, Brake Pedal Check and Adjustment .), and then go to Step 7.

STEP 5. Brake booster check
Refer to GROUP 35A - On-vehicle Service, Brake Booster Operating Check .

Q. Is the check result normal?
Go to Step 6.
After replacing the brake booster, go to Step 7.

STEP 6. Check whether the diagnosis code is reset.

Q. Is diagnosis code No.C121D set?
Replace the hydraulic unit (ASC-ECU) (Refer to ). Then go to Step 7.
The trouble can be an intermittent malfunction (Refer to GROUP 00 - How to Cope with Intermittent Malfunction ).

STEP 7. Check whether the diagnosis code is reset.

Q. Is diagnosis code No.C121D set?
Return to Step 1.
This diagnosis is complete.