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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 DTC may be stored. Prior to this diagnosis, diagnose the CAN bus lines (Refer to GROUP 54C - CAN Bus Line Diagnostic Flow ).
  • 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 (Refer to , and ).
  • 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 stores this DTC.
 
 
 
PROBABLE CAUSES
  • Incorrect brake pedal height
  • Incorrect adjustment of the stoplight switch
  • Master cylinder malfunction
  • Brake booster malfunction
  • ASC-ECU malfunction
 
 
 
 
DIAGNOSIS
STEP 1. Using scan tool MB991958, diagnose the CAN bus lines.
Use scan tool to diagnose the CAN bus lines.
Is the check result normal?
Go to Step 3.
Repair the CAN bus lines (Refer to GROUP 54C - CAN Bus Diagnostics Table ). On completion, go to Step 2.
 
STEP 2. DTC recheck after resetting CAN bus lines
Is DTC C121D stored?
Go to Step 3.
The procedure is complete.
 
STEP 3. Brake pedal check
Refer to GROUP 35A - On-vehicle Service/Brake Pedal Inspection .
Is the check result normal?
Go to Step 4.
Go to GROUP 35A - On-vehicle Service/Brake Pedal Inspection .
 
STEP 4. Check the installation condition of the stoplight switch.
Refer to GROUP 35A - On-vehicle Service/Brake Pedal Inspection .
Is the check result normal?
Go to Step 5.
Install the stop light switch correctly (Refer to GROUP 35A - On-vehicle Service/Brake Pedal Inspection ), and then go to Step 7.
 
STEP 5. Brake booster check
Refer to GROUP 35A - On-vehicle Service/Brake Booster Operating Check .
Is the check result normal?
Go to Step 6.
After replacing the brake booster (Refer to GROUP 35A - Master Cylinder Assembly and Brake Booster ), go to Step 7.
 
STEP 6. Check whether the DTC is stored again.
Erase the DTC.
Is DTC C121D stored?
Replace the hydraulic unit (integrated with ASC-ECU) (Refer to ).
The trouble can be an intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/How to Cope with Intermittent Malfunctions ).
 
STEP 7. Check whether the DTC is stored again.
Erase the DTC.
Is DTC C121D stored?
Return to Step 1.
The procedure is complete.