p0455 2003 chevy silverado

My 2003 has been setting a p0455 code since I bought it a year ago. The previous owner had taken it to the dealer and they kept telling him it was the gas cap.
I've replaced the cap,checked all the vacuum lines and canisters and can find nothing amiss. If I reset the mil and keep the tank above 3/4 full, the code does not set...but if it drops below 1/2 the code will set again. Anyone have a flow chart or any other method to track this down? Thanks

I'm not sure why your light doesn't come on with the tank above 3/4
On some chryslers with a full tank the evap system tests don't throw a code that will turn the light on because the rationale in the pcm is that the system has to allow for expansion through changing temperatures etc.
Maybe your GM is similar, the pcm still runs the test but it often doesn't turn on the light if there is a leak sensed because of the fuel level. Some
possible causes for yours could still be the purge valve/solenoid, I know that's a common failure on sunfires and cavaliers but as well thing to check are still the gas cap, filler neck, rollover valve in the top of the fuel tank etc. The easiest way to test for evap leaks is with a evap emission smoke machine but it's not a tool many do it yourselfers have. One last thing some software updates done at the dealer might address this same concern if it's a situation where you have what they call an erroneous code, meaning it's there but there's no symptons. Just be sure not to overfill the gas tank when fuelling or you can create similar propblems

Good luck

Jeff

Thanks jeff.