C121E
DTC C121E: Abnormality in brake fluid pressure sensor output signalCAUTION:
^ 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.
^ 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
This diagnostic trouble codes will be set 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 stop lamp switch
^ Stop lamp switch malfunction
^ Brake drag
^ 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
STEP 1. M.U.T.-III CAN bus diagnostics
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. On completion, go to Step 2.
STEP 2. Diagnostic trouble code recheck after resetting CAN bus lines
Q: Is DTC C121E set?
YES: Go to Step 3.
NO: The procedure is complete.
STEP 3. Brake pedal check
Refer to BRAKE PEDAL INSPECTION. Brake Pedal Inspection
Q: Is the check result normal?
YES: Go to Step 4.
NO: Go to BRAKE PEDAL INSPECTION. Brake Pedal Inspection
STEP 4. Check whether the diagnostic trouble code is set.
Refer to Diagnosis Function [Rear combination light].
Q: Is any DTC set?
YES: Repair or replace the rear combination light or rear combination light circuit.
NO: Go to Step 5.
STEP 5. Check for stop light switch installation
Q: Is the check result normal?
YES: Go to Step 6.
NO: Install the stop light switch correctly, and then go to Step 10.
STEP 6. Stop light switch continuity check
1) Remove the stop light switch.
2) Connect the circuit tester (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.
Q: Is the check result normal?
YES: Go to Step 7.
NO: Replace the stop light switch, and then go to Step 10.
STEP 7. Brake drag check
Check the brake system for drag.
Q: Is the check result normal?
YES: Go to Step 8.
NO: Repair the brake drag, and then go to Step 10.
STEP 8. Brake booster check
Refer to BRAKE BOOSTER OPERATION CHECK. Testing and Inspection
Q: Is the check result normal?
YES: Go to Step 9.
NO: Replace the brake booster, and then go to Step 10.
STEP 9. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121E set?
YES: Replace the ASC-ECU. Then go to Step 10.
NO: The trouble can be an intermittent malfunction.
STEP 10. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121E set?
YES: Return to Step 1.
NO: The procedure is complete.