C121D

DTC C121D: Abnormality in brake fluid pressure sensor circuit

CAUTION:
^ 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.

CIRCUIT OPERATION
The brake fluid 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 outputs it.

DTC SET CONDITIONS
When the pressure sensor output signal is not within the standard value range, ASC-ECU outputs this diagnostic trouble code.

PROBABLE CAUSES
^ Incorrect adjustment of brake pedal height
^ Master cylinder malfunction
^ Brake booster malfunction
^ 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 A
^ MB991997: ASC check 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 2.
NO: Repair the CAN bus lines. On completion, go to Step 2.

STEP 2. Brake pedal check
Q: Is the check result normal?
YES: Go to Step 3.
NO: Adjust the brake pedal, and then go to Step 3.

STEP 3. Brake booster check
Q: Is the check result normal?
YES: Go to Step 4.
NO: Replace the brake booster.

STEP 4. Check whether the diagnostic trouble code is reset.
Q: Is DTC C121D set?
YES: Replace the ASC-ECU.
NO: The procedure is complete.