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 Initial Inspection and Diagnostic Overview ).
- 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 G and Yaw Rate Sensor Calibration , Testing and Inspection , Master Cylinder Brake Fluid Pressure Sensor Calibration , and Testing and Inspection ).

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?

YES Go to Step 3.
NO Repair the CAN bus lines CAN Bus Diagnostics Chart).
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?

YES Go to Step 3.
NO The procedure is complete.
STEP 3. Brake pedal check

Brake Pedal Inspection.

Q. Is the check result normal?

YES Go to Step 4.
NO Adjust the brake pedal height Brake Pedal Inspection). Then go to Step 7.
STEP 4. Check of stoplight switch installation status

Brake Pedal Inspection.

Q. Is the check result normal?

YES Go to Step 5.
NO Install the stoplight switch correctly Brake Pedal Inspection). Then go to Step 7.
STEP 5. Brake booster check

Testing and Inspection.

Q. Is the check result normal?

YES Go to Step 6.
NO Replace the brake booster Service and Repair). Then go to Step 7.
STEP 6. Check whether the DTC is reset.

Erase the DTC.

Q. Is DTC C121D set?

YES Replace the hydraulic unit (integrated with ASC-ECU) (Refer to Service and Repair).
Then go to Step 7.
NO Intermittent malfunction How to Cope With Intermittent Malfunctions).
STEP 7. Check whether the DTC is reset.

Erase the DTC.

Q. Is DTC C121D set?

YES Return to Step 1.
NO The procedure is complete.