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, Trouble code diagnosis ).
  • 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 outputs this diagnosis code.

PROBABLE CAUSES



  • Incorrect adjustment of brake pedal height
  • Master cylinder malfunction
  • Brake booster malfunction
  • ASC-ECU malfunction

DIAGNOSIS PROCEDURE


STEP 1. M.U.T.-III CAN bus diagnosis


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

Q. Is the check result normal?

Go to Step 2.
Repair the CAN bus lines (Refer to GROUP 54C - Troubleshooting ). On completion, go to Step 2.

STEP 2. Brake pedal check


Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?

Go to Step 3.
Adjust the brake pedal, and then go to Step 3.

STEP 3. Brake booster check


Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?

Go to Step 4.
After replacing the brake booster, go to Step 4.

STEP 4. Check whether the diagnosis code is reset.


Q. Is the diagnosis code No. C121D set?

Replace the ASC-ECU.
This diagnosis is complete.