DTC C121D: Abnormality in brake fluid pressure sensor circuit: Diagnosis

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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 C121D 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 7.
STEP 4. Check of stoplight switch installation status
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 7.
STEP 5. Brake booster check
Refer to ON-VEHICLE SERVICE .

Q: Is the check result normal?
1. YES: Go to Step 6.
2. NO: Replace the brake booster. (Refer to MASTER CYLINDER ASSEMBLY AND BRAKE BOOSTER .) Then go to Step 7.
STEP 6. Check whether the DTC is reset.
Q: Is DTC C121D set?
1. YES: Replace the hydraulic unit (incorporates in ASC-ECU). (Refer to HYDRAULIC UNIT .) Then go to Step 7.
2. NO: Intermittent malfunction. (Refer to HOW TO COPE WITH INTERMITTENT MALFUNCTION .)
STEP 7. Check whether the DTC is reset.
Q: Is DTC C121D set?
1. YES: Return to Step 1.
2. NO: The procedure is complete.