I would look for a large vacuum leak in the line to the Canister Purge Solenoid. You need to find something that would cause both banks to be getting too much air or not enough fuel. Since you don't mention any driveability issues, I would assume that fuel delivery isn't the first area to check. I think excess air is the most likely cause. For the evaporative monitor to run, the PCM commands the canister vent solenoid to close, sealing the system and then commands the purge solenoid on to pull a vacuum on the vent side of the fuel system. It then monitors the fuel tank pressure to determine if there is a leak. The 455 code means that the computer didn't see any or very little pressure change in the fuel tank pressure. This is my rationale for closely checking for a vaccuum leak in the purge line, this should be a large vaccum hose that connects near the throttle body-runs to the purge control solenoid or valve and then runs back to the fuel tank. If it isn't obvious you might get a length of fuel line or something similar and hold one end to your ear and work along the line listening for a whistle. The other option is to take it to a shop and have them diagnose the problem. I would use my smoke machine to check for the leak, expensive machine but speeds things up alot.
2007 GT/CS, Vista Blue, FRP 200hp supercharger, Ford Racing GTA mufflers, Boss 302 Quad Exhaust,GT-500 rear spoiler, FRP short throw shifter. Eibach Mutli-Pro Coil Over Suspension, BMR K Member & Adjustable Lower Control Arms, Steeda Bump Steer Kit, Steeda X-5 Ball Joints,Maximum Caster Camber Plates(required maching my Steeda Billet strut tower brace end mounts), CHE Adjustable Lower Contols Arms and Anti Squat Brackets. GT-500 wheels and tires, Ford Racing 14" Brembo brake set-up.