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DTC C121D 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 GROUP 54C - CAN Bus Diagnostics Table ).
  • Whenever ECU is replaced, ensure that the CAN bus lines are normal.
 
 
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 outputs 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. Brake pedal check
Refer to GROUP 35A - On-vehicle Service, Brake Pedal Check .
Is the check result normal?
Go to Step 2.
Repair it.
 
STEP 2. Check for stoplight switch installation
Refer to GROUP 35A - On-vehicle Service, Brake Pedal Check .
Is the check result normal?
Go to Step 3.
Repair it.
 
STEP 3. Brake booster check
Refer to GROUP 35A - On-vehicle Service/Brake Booster Operation Check .
Is the check result normal?
Go to Step 4.
Replace the brake booster (Refer to GROUP 35A - Master Cylinder Assembly and Brake Booster ).
 
STEP 4. Check whether the DTC is set again.
(1) Erase the DTC.
(2) Turn the ignition switch to the "LOCK" (OFF) position.
(3) Turn the ignition switch to the "ON" position.
Is DTC C121D set?
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 ).