Fuel Tank Pressure Sensor
WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The FTP sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The Powertrain Control Module (PCM) provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the PCM that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure = low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum = high voltage.