DTC C1581: Brake Boost Negative Pressure Abnormal

DTC detecting condition: 

This malfunction is stored if vacuum pressure value inside brake booster drops below the normal pressure and stays for a specific period of time.

Trouble symptom: 

Brake pedal becomes heavier. (Brake assist function is activated when vacuum pressure drops.)

1. CHECK DTC  .
   1. Start the engine.
   2. Warm up for at least three minutes.
   3. Read the DTC of [Brake Control] using the Subaru Select Monitor. Ref. to BRAKE CONTROL (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) 

   Is DTC C1581 a current code?

   Yes  : Go to  2.

   No  : Currently, it is normal. It results from a temporary vacuum pressure drop.

2. CHECK OUTPUT OF BRAKE BOOSTER PRESSURE SENSOR WITH SUBARU SELECT MONITOR  .

   Using the Subaru Select Monitor, read [Brake Booster Gauge Sensor Output] in [Data Monitor] of [Brake Control]. Ref. to BRAKE CONTROL (DIAGNOSTICS)>DATA MONITOR 

   Is [Brake Booster Gauge Sensor Output] -20 kPa or more?

   Yes  : Go to  3.

   No  : Currently, it is normal. It results from a temporary vacuum pressure drop.

3. CHECK OUTPUT OF BRAKE BOOSTER PRESSURE SENSOR WITH SUBARU SELECT MONITOR  .

   Using Subaru Select Monitor, read [Brake Booster Gauge Sensor Output] in [Data Monitor] of [Brake Control] while depressing the brake pedal several times. Ref. to BRAKE CONTROL (DIAGNOSTICS)>DATA MONITOR 

   Does the value of [Brake Booster Gauge Sensor Output] change along with the operation?

   Yes  : Currently, it is normal. It results from a temporary vacuum pressure drop.

   No  : Go to  4.

4. CHECK BRAKE VACUUM SYSTEM  .
   1. Check for connections and air tightness of brake vacuum system (between brake booster and intake manifold).
   2. Check for connections and air tightness of brake vacuum system (between brake booster and brake vacuum pump).

   Are there any air leaks in the brake vacuum system?

   Yes  : Repair the air leak in the brake vacuum system.

   No  : Replace the brake booster. Ref. to BRAKE>BRAKE BOOSTER