2004 Deville V8 -P0455
 

Hi, I have a repair shop and this vehicle came with this code. The gas cap had already been replaced and we did the same to make sure. We used a smoke machine to check for leaks and found that the canister vent valve (located in the rear) was stuck open when the car was on.
For those who don't know, it open when the car is off to let out vapors and closes when the car is on to create a vacuum. If it stays open then the vehicle will detect a large leak.
We replaced the valve and a few days later the code reappeared. Another smoke test later and we find that the canister vent valve was still open. We checked the power of the cables and its fuse, and they checked out. Where should I go next? It it possible that the PCM is having problems since that's what controls the vent valve? I was thinking of this but the PCM is showing no signs of malfunction. The car is working fine.