DTC C121E: Abnormality in brake fluid pressure sensor output signal: 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

STEP 1. Using scan tool MB991958, diagnose the CAN bus lines.
Use scan tool to 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 CAN BUS DIAGNOSTICS TABLE ). On completion, go to Step 2.
STEP 2. DTC recheck after resetting CAN bus lines
Q: Is DTC C121E set?
1. YES: Go to Step 3.
2. NO: The procedure is complete.
STEP 3. Brake pedal check
Refer to BRAKE PEDAL CHECK AND ADJUSTMENT .

Q: Is the check result normal?
1. YES: Go to Step 4.
2. NO: Adjust the brake pedal height. (Refer to BRAKE PEDAL CHECK AND ADJUSTMENT ). 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 ), and 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 terminals No. 1 and 2.
3. When no continuity is detected with the plunger pressed from the edge of the outer case by the dimension shown in the illustration and when continuity is detected with the plunger released, the stoplight 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 ), and then go to Step 9.
  
  Fig 1: Connecting Circuit Tester To Stoplight Switch Connector Terminals
  Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
STEP 6. Brake drag check
Check the brake system for drag.

Q: Is the check result normal?
1. YES: Go to Step 7.
2. NO: Repair the brake drag, and then go to Step 9.
STEP 7. Brake booster check
Refer to BRAKE BOOSTER OPERATION CHECK .

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 < Except RALLIART >, < RALLIART > ), and then go to Step 9.
STEP 8. Check whether the DTC is reset.
Erase the DTC.

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: The trouble can be an intermittent malfunction (Refer to HOW TO COPE WITH INTERMITTENT MALFUNCTION ).
STEP 9. Check whether the DTC is reset.
Erase the DTC.

Q: Is DTC C121E set?
1. YES: Return to Step 1.
2. NO: The procedure is complete.