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DTC C121D: Abnormality in brake fluid pressure sensor circuit


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 hydraulic unit (integrated with ASC-ECU) is replaced, 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
The pressure 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 then outputs the signal.

DTC SET CONDITIONS
If the pressure sensor output signal is outside the standard value range, this diagnostic trouble code is set.

PROBABLE CAUSES

  • Incorrect brake pedal height
  • Incorrect adjustment of stoplight switch
  • Master cylinder malfunction
  • Brake booster 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
  • MB991997: ASC check harness

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 C121D set?
Go to Step 3.
The procedure 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 height. (Refer to GROUP 35A - On-vehicle Service .) Then go to Step 7.

STEP 4. Check of stoplight switch installation status
Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?
Go to Step 5.
Install the stoplight switch correctly. (Refer to GROUP 35A - On-vehicle Service .) Then go to Step 7.

STEP 5. Brake booster check
Refer to GROUP 35A - On-vehicle Service .

Q. Is the check result normal?
Go to Step 6.
Replace the brake booster. (Refer to GROUP 35A - Master Cylinder Assembly and Brake Booster .) Then go to Step 7.

STEP 6. Check whether the DTC is reset.

Q. Is DTC C121D set?
Replace the hydraulic unit (incorporates in ASC-ECU).(Refer to .) Then go to Step 7.
Intermittent malfunction. (Refer to GROUP 00 - How to Cope with Intermittent Malfunction .)

STEP 7. Check whether the DTC is reset.

Q. Is DTC C121D set?
Return to Step 1.
The procedure is complete.