C121D

DTC C121D Abnormality in brake fluid pressure sensor circuit

CAUTION
^ If there is any problem in the CAN bus lines, an incorrect DTC may be set. Prior to this diagnosis, diagnose the CAN bus lines (Refer to Diagnostic trouble code diagnosis Diagnostic Trouble Code Descriptions).
^ Whenever ECU is replaced, ensure that the CAN bus lines are normal.
^ 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.

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.

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

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

STEP 1. Using scan tool MB991958, diagnose the CAN bus lines.
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 (Refer to Diagnosis - CAN Bus Diagnostic Table CAN Bus Diagnostics Table). On completion, go to Step 2.

STEP 2. DTC recheck after resetting CAN bus lines
Q: Is DTC C121D set?
YES: Go to Step 3.
NO: The procedure is complete.

STEP 3. Brake pedal check

Q: Is the check result normal?
YES: Go to Step 4.
NO: Go to On-vehicle Service. Then go to Step 7.

STEP 4. Check the installation condition of the stoplight switch.
Q: Is the check result normal?
YES: Go to Step 5.
NO: Install the stoplight switch correctly, and then go to Step 7.

STEP 5. Brake booster check
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 DTC is reset.
Q: Is DTC C121D set?
YES: Replace the hydraulic unit (ASC-ECU). Then go to Step 7.
NO: The trouble can be an intermittent malfunction.

STEP 7. Check whether the DTC is reset.
Q: Is DTC C121D set?
YES: Return to Step 1.
NO: The procedure is complete.