The Powertrain Control Module (PCM) monitors the amount of vacuum/pressure in the EVAP system by monitoring the fuel tank pressure sensor signal. The PCM seals the EVAP system by commanding the EVAP Purge Solenoid OFF and the EVAP vent valve ON. The PCM monitors the fuel tank pressure sensor in order to determine if the EVAP Purge Solenoid is leaking vacuum to the EVAP system. In most cases the repair is installing a new purge solenoid.
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