The pressure sensor is inside the gas tank. It can be tested with a DRB III or similar scan tool by removing the regular gas cap, screwing on a test cap and thru that cap slightly raising pressure and observe result via scan tool. I doubt that is your problem. More than likely you have a faulty shut off valve on the vapor canister. Probably easier to replace entire canister ( two available on ebay for a little over $100 )
You could also try a smoke test and see if you observe a leak near the vapor canister.

PS The car tests the EVAP system by creating a vacuum thru the purge valve in the engine bay and shutting the shut off valve on the canister then reading the pressure switch located in the fuel tank.