Thought I would share my experience...My CEL was coming on with an associated code of P0446 for a couple of months - at first after a fillup and then a couple of time per tank. I did some digging around, and found this very good document that describes the fuel and emissions control system, and my intent was to follow some of the diagnostic routines. I tried replacing the cap gasket first, but that didn't change anything. I then saw some mention in another dealer document that listed user error as a possible cause of this error - they found that some people were not fully inserting the fuel nozzle into the filler pipe, and that this was causing the error. When I thought about my own behaviour, I realized that I was guilty of this - when fueling I was only inserting the nozzle part way into the filler pipe, and there was air space around the nozzle. Don't know why I was doing this - maybe I always have! But particularly with the Prius where there is a flexible bladder, the fuel pressure helps to expand the bladder, and it is important that the seal around the fuel nozzle be a good as possible. There is a rubber gasket that seals around the nozzle when it is fully inserted. Bottom line: I started fully inserting the nozzle into the filler neck during fillups, and haven't had a reoccurence since the second fillup using this method. It's been several months since I started doing this, and I haven't had any further errors. Problem solved!
