Mitsubishi Lancer Evolution X. Manual — part 168

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-137

DTC C121D: Abnormality in brake fluid pressure sensor circuit

CAUTION

• If there is any problem in the CAN bus lines, an incor-

rect 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 pres-
sure sensor, cut valve, and inlet valve.(Refer to

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

If the pressure sensor output signal is outside the standard
value range, this diagnostic trouble code is set.

PROBABLE CAUSES

• Incorrect brake pedal height
• Incorrect adjustment of stoplight switch
• 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. 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 GROUP 54C −

Troubleshooting

P.54C-15

.) After repairing the CAN

bus line, go to Step 2.

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-138

STEP 2. DTC recheck after repairing the CAN bus line
Q: Is DTC C121D set?

YES : Go to Step 3.
NO : The procedure is complete.

STEP 3. Brake pedal check
Refer to GROUP 35A − On-vehicle Service

P.35A-12

Q: Is the check result normal?

YES : Go to Step 4.
NO : Adjust the brake pedal height. (Refer to GROUP 35A −

On-vehicle Service

P.35A-12

.) Then go to Step 7.

STEP 4. Check of stoplight switch installation status
Refer to GROUP 35A − On-vehicle Service

P.35A-12

Q: Is the check result normal?

YES : Go to Step 5.
NO : Install the stoplight switch correctly. (Refer to GROUP

35A − On-vehicle Service

P.35A-12

.) Then go to Step

STEP 5. Brake booster check
Refer to GROUP 35A − On-vehicle Service

P.35A-15

Q: Is the check result normal?

YES : Go to Step 6.
NO : Replace the brake booster. (Refer to GROUP 35A −

Master Cylinder Assembly and Brake Booster

P.35A-26

.) Then go to Step 7.

STEP 6. Check whether the DTC is reset.
Q: Is DTC C121D set?

YES : Replace the hydraulic unit (incorporates in

ASC-ECU).(Refer to

P.35C-298

.) Then go to Step 7.

NO : Intermittent malfunction. (Refer to GROUP 00 − How

to Cope with Intermittent Malfunction

P.00-15

STEP 7. Check whether the DTC is reset.
Q: Is DTC C121D set?

YES : Return to Step 1.
NO : The procedure is complete.

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-139

DTC C121E: Abnormality in brake fluid pressure sensor output signal

CAUTION

• If there is any problem in the CAN bus lines, an incor-

rect 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 pres-
sure sensor, cut valve, and inlet valve.(Refer to

CIRCUIT OPERATION

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

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-140

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 GROUP 54C −

Troubleshooting

P.54C-15

.) 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 GROUP 35A − On-vehicle Service

P.35A-12

Q: Is the check result normal?

YES : Go to Step 4.
NO : Adjust the brake pedal height. (Refer to GROUP 35A −

On-vehicle Service