Mitsubishi Outlander GS45X. Manual - part 426

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-131

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 (Refer to 
GROUP 54C 

− CAN Bus Line Diagnostic Flow 

P.54C-10

).

• When the hydraulic unit (integrated with ASC-ECU) is 

replaced, always carry out the calibration of the steer-
ing wheel sensor, the G and yaw rate sensor and brake 
fluid pressure sensor (Refer to 

P.35C-282

, 

P.35C-283

 

and 

P.35C-284

).

• 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 stan-
dard value range, ASC-ECU outputs this diagnostic trouble 
code.

.

PROBABLE CAUSES

• Incorrect brake pedal height

• Incorrect adjustment of the 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

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?

YES : Go to Step 3.
NO :  Repair the CAN bus lines (Refer to GROUP 54C 

− 

CAN Bus Diagnostics Table 

P.54C-17

). On 

completion, go to Step 2.

DIAGNOSIS

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ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-132

STEP 2. DTC recheck after resetting CAN bus lines
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/Brake Pedal 

Inspection 

P.35A-15

.

Q: Is the check result normal?

YES : Go to Step 4.
NO :  Go to GROUP 35A 

− On-vehicle Service/Brake Pedal 

Inspection 

P.35A-15

.

STEP 4. Check the installation condition of the stoplight 
switch.
Refer to GROUP 35A 

− On-vehicle Service/Brake Pedal 

Inspection 

P.35A-15

.

Q: Is the check result normal?

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

35A 

− On-vehicle Service/Brake Pedal Inspection 

P.35A-15

), and then go to Step 7 .

STEP 5. Brake booster check
Refer to GROUP 35A 

− On-vehicle Service/Brake Booster 

Operating Check 

P.35A-17

.

Q: Is the check result normal?

YES : Go to Step 6.
NO :  After replacing the brake booster (Refer to GROUP 

35A 

− Master Cylinder Assembly and Brake Booster 

P.35A-30

), go to Step 7.

STEP 6. Check whether the diagnostic trouble code is 
reset.
Erase the DTC.
Q: Is DTC C121D set?

YES : Replace the hydraulic unit (integrated with ASC-ECU) 

(Refer to 

P.35C-287

).

NO :  The trouble can be an intermittent malfunction (Refer 

to GROUP 00 

− How to Use Troubleshooting/How to 

Cope with Intermittent Malfunctions 

P.00-15

).

STEP 7. Check whether the diagnostic trouble code is 
reset.
Erase the DTC.
Q: Is DTC C121D set?

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

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-133

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 (Refer to 
GROUP 54C 

− CAN Bus Line Diagnostic Flow 

P.54C-10

).

• Whenever ECU is replaced, ensure that the CAN bus 

lines are normal.

• When the hydraulic unit (integrated with ASC-ECU) is 

replaced, always carry out the calibration of the steer-
ing wheel sensor, the G and yaw rate sensor and brake 
fluid pressure sensor (Refer to 

P.35C-282

, 

P.35C-283

 

and 

P.35C-284

).

.

OPERATION

The hydraulic 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

This diagnostic trouble codes will be set under the cases 
below:

• When the pressure sensor offset is not within the standard 

value range

• When 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

• Stoplight switch malfunction

• Brake drag

• 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

DIAGNOSIS

TSB Revision

ACTIVE STABILITY CONTROL SYSTEM (ASC)

35C-134

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 3.
NO :  Repair the CAN bus lines (Refer to GROUP 54C 

− 

CAN Bus Diagnostics Table 

P.54C-17

). On 

completion, go to Step 2.

STEP 2. Diagnostic trouble code recheck after resetting 
CAN bus lines
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/Brake Pedal 

Inspection 

P.35A-15

.

Q: Is the check result normal?

YES : Go to Step 4.
NO :  Go to GROUP 35A 

− On-vehicle Service/Brake Pedal 

Inspection 

P.35A-15

.

STEP 4. Check whether the other system diagnostic 
trouble code is set.
Refer to GROUP 54A 

− Rear Combination Light/Diagnostic 

Trouble Code chart 

P.54A-228

.

Q: Is any DTC set?

YES : Repair or replace the rear combination light or rear 

combination light circuit.

NO :  Go to Step 5.

STEP 5. Check for stoplight switch installation
Refer to GROUP 35A 

− On-vehicle Service/Brake Pedal 

Inspection 

P.35A-15

.

Q: Is the check result normal?

YES : Go to Step 6.
NO :  Install the stop light switch correctly (Refer to GROUP 

35A 

− On-vehicle Service/Brake Pedal Inspection 

P.35A-15

), and then go to Step 10 .