If the PCM senses that there is no leak in the system, it will run the purge monitor. This test is completed by calculating or measuring the amount of vapors that are being purged. In many systems, the PCM calculates the purge flow based on MAF or MAP sensor data and compares it to FTP or fuel trim data. When fuel trim is used, the purge monitor determines if the purge system is functioning properly by applying a long duty cycle to the purge solenoid. The subsequent change in STFT is monitored. If the system is working correctly as the duty cycle of the solenoid is increased, the STFT should change proportionately. This is the most commonly used method for checking the operation of the purge system