Code No. C121E Abnormality in brake fluid pressure sensor output signal


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
This diagnosis codes will be set under the cases below:

  • When the pressure sensor offset is not within the standard value range
  • When the estimated pressure sensor temperature is not normal

PROBABLE CAUSES

  • Incorrect adjustment of brake pedal height
  • Master cylinder malfunction
  • Brake booster malfunction
  • Incorrect installation position of stop lamp switch
  • Malfunction of the stop lamp switch
  • Brake drag
  • 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 3.
Repair the CAN bus lines (Refer to GROUP 54C - Troubleshooting ). On completion, go to Step 2.

STEP 2. Diagnosis code recheck after resetting CAN bus lines

Q. Is the diagnosis code No. C121E set?
Go to Step 3.
This diagnosis is complete.

STEP 3. Brake pedal check
Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?
Go to Step 4.
Adjust the brake pedal, and then go to Step 4.

STEP 4. Check for stop lamp switch installation
Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?
Go to Step 5.
Install the stop lamp switch correctly, and then go to Step 5.

STEP 5. Stop lamp switch continuity check
(1)Remove the stop lamp switch. (Refer to GROUP 35A -Brake Pedal .)

(2)Connect the circuit tester (Ω range) to the stop lamp switch connector terminals No. 1 and 2.
(3)When no continuity is detected with the plunger pressed from the edge of the outer case by the dimension shown in the figure and when continuity is detected with the plunger released, the stop lamp switch is in good condition.

Q. Is the check result normal?

Go to Step 6.

Replace the stop lamp switch, and then go to Step 6.

STEP 6. Brake drag check
Check the brake system for drag.

Q. Is the check result normal?
Go to Step 7.
Repair the brake drag.

STEP 7. Brake booster check
Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?
Go to Step 8.
Replace the brake booster.

STEP 8. Check whether the diagnosis code is reset.

Q. Is the diagnosis code No. C121E set?
Replace the ASC-ECU.
This diagnosis is complete.