C121D

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.
^ 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.
^ Whenever ECU is replaced, ensure that the CAN bus lines are normal.

CIRCUIT OPERATION
The brake fluid 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 outputs it.

DTC SET CONDITIONS
When the pressure sensor output signal is not within the standard value range, ASC-ECU outputs this diagnostic trouble code.

PROBABLE CAUSES
^ Incorrect brake pedal height
^ Incorrect adjustment of the stop lamp 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

STEP 1. M.U.T.-III CAN bus diagnostics
Use scan tool to diagnose the CAN bus lines.
Q: Is the check result normal?
YES: Go to Step 3.
NO: Repair the CAN bus lines. On completion, go to Step 2.

STEP 2. Diagnostic trouble code recheck after resetting CAN bus lines
Q: Is DTC C121D set?
YES: Go to Step 3.
NO: This diagnosis is complete.

STEP 3. Brake pedal check
Refer to BRAKE PEDAL INSPECTION. Brake Pedal Inspection
Q: Is the check result normal?
YES: Go to Step 4.
NO: Go to BRAKE PEDAL INSPECTION. Brake Pedal Inspection

STEP 4. Check the installation condition of the stop light switch.
Refer to BRAKE PEDAL INSPECTION. Brake Pedal Inspection
Q: Is the check result normal?
YES: Go to Step 5.
NO: Install the stop light switch correctly, and then go to Step 7.

STEP 5. Brake booster check
Refer to BRAKE BOOSTER OPERATION CHECK. Testing and Inspection
Q: Is the check result normal?
YES: Go to Step 6.
NO: After replacing the brake booster, go to Step 7.

STEP 6. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121D set?
YES: Replace the ASC-ECU.
NO: The trouble can be an intermittent malfunction.

STEP 7. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121D set?
YES: Return to Step 1.
NO: This diagnosis is complete.