FWD DeVille 1985-2005, Fleetwood, and 60 Special
My 99 kept throwing gas cap codes. So i replaced it three times. It finally dawned on me that the cap was not sealing properly. It was not the caps problem, it was the dirt and some debris that had built up on the filler neck.
So I removed the cap and stuffed a new clean rag in the filler neck...