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 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.
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
This diagnostic trouble codes will be set under the cases below:
^ The pressure sensor offset is outside the specified range. (Pressure is generated at all times)
^ 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
^ Malfunction of the stoplight switch
^ Brake drag
^ Malfunction of the ASC-ECU
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. (Refer to Troubleshooting Can Bus Diagnostics Table.) After repairing the CAN bus line, go to Step 2.
STEP 2. DTC recheck after repairing the CAN bus line
Q: Is DTC C121E set?
YES: Go to Step 3.
NO: The procedure is complete.
STEP 3. Brake pedal check
Refer to On-vehicle Service Brake Pedal Check and Adjustment.
Q: Is the check result normal?
YES: Go to Step 4.
NO: Adjust the brake pedal height. Then go to Step 9.
STEP 4. Check for stoplight switch installation
Refer to On-vehicle Service Brake Pedal Check and Adjustment.
Q: Is the check result normal?
YES: Go to Step 5.
NO: Install the stoplight switch correctly. Then go to Step 9.
STEP 5. Stoplight switch continuity check
1) Remove the stoplight switch.
2) Connect the circuit tester (Ohms range) to the stoplight switch connector terminal Nos. 1 and 2.
3) When continuity does not exist with the plunger pressed from the outer case end face by the dimension shown in the figure, and when continuity exists with the plunger released, the switch is in good condition.
Q: Is the check result normal?
YES: Go to Step 6.
NO: Replace the stoplight switch. Then go to Step 9.
STEP 6. Brake drag check
Check the brake drag force. (Refer to On-vehicle Service Brake Drag Force Check.)
Q: Is the check result normal?
YES: Go to Step 7.
NO: Adjust the brake drag, and then go to Step 9.
STEP 7. Brake booster check
Refer to On-vehicle Service Testing and Inspection.
Q: Is the check result normal?
YES: Go to Step 8.
NO: Replace the brake booster. Then go to Step 9.
STEP 8. Check whether the DTC is reset.
Q: Is DTC C121E set?
YES: Replace the hydraulic unit (incorporates in ASC-ECU). Then go to Step 9.
NO: Intermittent malfunction.
STEP 9. Check whether the DTC is reset.
Q: Is DTC C121E set?
YES: Return to Step 1.
NO: The procedure is complete.