Anthony C ----- "The ECU sends 12 volts to the canister shut off valve causing it to open" That is not correct.

The canister's shut off value normal position is open. The PCM sends a signal to close the shut off value when it tests the system by opening the purge value so the engine can suck (creates a vacuum) in the system while testing. There is a pressure sensor in the gasoline tank that senses the vacuum during the test.

PS a bad shut off value or clogged vent line often makes it difficult to fill the car with gas. I'm guessing you have one or the other. 1 Bad shut off value. 2 clogged vent line.