C109D Code: Meaning, Causes, and Symptoms | OBD2 Codes
C109D – Low Vacuum Condition Detected
C109D Possible Symptoms
Anti-Lock Brake System (ABS) Warning Light ON,Engine Light ON (or Service Engine Soon Warning Light)
C109D Possible Causes
Loose Brake Booster Vacuum Hose,Faulty Brake Booster Vacuum Sensor,Brake Booster Vacuum Sensor harness is open or shorted,Brake Booster Vacuum Sensor circuit poor electrical connection
The vacuum sensor is used by the Anti-Lock Brake System (ABS) Module to determine if hydraulic pump intervention is necessary due to loss of vacuum in the booster. The ABS module sends a 5-volt reference voltage to the sensor, the sensor compares the pressure in the booster to the ambient air pressure in the engine compartment and sends a varying voltage back to the ABS module based on the difference between the pressures. Normally, the pressure in the booster is much lower than ambient air pressure. As the pressure in the booster gets higher (closer to matching ambient air pressure), the lower the voltage sent back to the ABS module becomes. When the vacuum in the brake booster falls below a predetermined level, the ABS module will command the hydraulic pump motor to activate on the next (and any subsequent) brake pedal application to assist with vehicle braking. The ABS module will continue to use the pump motor for braking assist as long as the low vacuum condition exists.