DTC C121E: Abnormality In Brake Fluid Pressure Sensor Output Signal: Diagnosis

WARNING: This page is about a different car, the 2012 Mitsubishi Lancer. However, it is still accessible from the selected car via links, so may be relevant.

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?
1. YES: Go to Step 3.
2. NO: Repair the CAN bus lines. (Refer to TROUBLESHOOTING .) 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?
1. YES: Go to Step 3.
2. NO: The procedure is complete.
STEP 3. Brake pedal check
Refer to ON-VEHICLE SERVICE .

Q: Is the check result normal?
1. YES: Go to Step 4.
2. NO: Adjust the brake pedal height. (Refer to ON-VEHICLE SERVICE .) Then go to Step 9.
STEP 4. Check for stoplight switch installation
Refer to ON-VEHICLE SERVICE .

Q: Is the check result normal?
1. YES: Go to Step 5.
2. NO: Install the stoplight switch correctly. (Refer to ON-VEHICLE SERVICE .) Then go to Step 9.
STEP 5. Stoplight switch continuity check
1. Remove the stoplight switch. (Refer to BRAKE PEDAL .)
2. Connect the circuit tester (Ω 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 illustration, and when continuity exists with the plunger released, the switch is in good condition.
  Q: Is the check result normal?
  1. YES: Go to Step 6.
  2. NO: Replace the stoplight switch. (Refer to BRAKE PEDAL .) Then go to Step 9.
    
    Fig 1: Checking Stoplight Switch Continuity
    Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
STEP 6. Brake drag check
Check the brake drag force. (Refer to ON-VEHICLE SERVICE .)

Q: Is the check result normal?
1. YES: Go to Step 7.
2. NO: Adjust the brake drag, and then go to Step 9.
STEP 7. Brake booster check
Refer to ON-VEHICLE SERVICE .

Q: Is the check result normal?
1. YES: Go to Step 8.
2. NO: Replace the brake booster. (Refer to MASTER CYLINDER ASSEMBLY AND BRAKE BOOSTER .) Then go to Step 9.
STEP 8. Check whether the DTC is reset.
Q: Is DTC C121E set?
1. YES: Replace the hydraulic unit (integrated with ASC-ECU). (Refer to HYDRAULIC UNIT .) Then go to Step 9.
2. NO: Intermittent malfunction. (Refer to HOW TO COPE WITH INTERMITTENT MALFUNCTION .)
STEP 9. Check whether the DTC is reset.
Q: Is DTC C121E set?
1. YES: Return to Step 1.
2. NO: The procedure is complete.