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DTC C121E: Abnormality in brake fluid pressure sensor output signal
 
 
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 ).
  • 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 (Refer to , and ).
 
 
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
This DTCs will be stored under the cases below:
  • When the pressure sensor offset is not within the standard value range
  • When the estimated pressure sensor temperature is not normal
 
 
 
PROBABLE CAUSES
  • Incorrect adjustment of brake pedal height
  • Master cylinder malfunction
  • Brake booster malfunction
  • Incorrect installation position of stoplight switch
  • Stoplight switch malfunction
  • Brake drag
  • ASC-ECU malfunction
 
 
 
 
DIAGNOSIS
STEP 1. Using scan tool MB991958, diagnose the CAN bus line.
Use scan tool MB991958 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 C121E 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 whether the other system DTC is stored.
Refer to GROUP 54A - Rear Combination Light/DTC chart .
Is any DTC stored?
Repair or replace the rear combination light or rear combination light circuit.
Go to Step 5.
 
STEP 5. Check for stoplight switch installation
Refer to GROUP 35A - On-vehicle Service/Brake Pedal Inspection .
Is the check result normal?
Go to Step 6.
Install the stop light switch correctly (Refer to GROUP 35A - On-vehicle Service/Brake Pedal Inspection ), and then go to Step 10.
 
STEP 6. Stoplight switch continuity check
(1) Remove the stoplight switch (Refer to GROUP 35A -Brake Pedal ).
(2) Connect the circuit tester (Ω range) to the stop light switch connector terminals No.1 and No.2.
(3) When no continuity is detected with the plunger pressed from the edge of the outer case by the dimension shown in the figure and when continuity is detected with the plunger released, the stop light switch is in good condition.
Is the check result normal?
Go to Step 7.
Replace the stop light switch (Refer to GROUP 35A -Brake Pedal ), and then go to Step 10.
 
STEP 7. Brake drag check
Check the brake system for drag (Refer to GROUP 35A - On-vehicle Service/Brake Drag Force Check ).
Is the check result normal?
Go to Step 8.
Repair the brake drag, and then go to Step 10.
 
STEP 8. Brake booster check
Refer to GROUP 35A - On-vehicle Service/Brake Booster Operation Check .
Is the check result normal?
Go to Step 9.
Replace the brake booster (Refer to GROUP 35A - Master Cylinder Assembly and Brake Booster ), and then go to Step 10.
 
STEP 9. Check whether the DTC is stored again.
Erase the DTC.
Is DTC C121E stored?
Replace the hydraulic unit (integrated with ASC-ECU) (Refer to ). Then go to Step 10.
The trouble can be an intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/How to Cope with Intermittent Malfunctions ).
 
STEP 10. Check whether the DTC is stored again.
Erase the DTC.
Is DTC C121E stored?
Return to Step 1.
The procedure is complete.