Info sourced online.
P0455 is a fairly straight forward OBD-II trouble code, in that it tells you that the engine computer has detected a large leak in the engine Evaporative Emissions Control (EVAP) system. The EVAP system prevents fuel vapors from escaping into the atmosphere. The system allows for fuel vapors from the fuel tank to be absorbed by charcoal pellets located in the charcoal canister (also known as EVAP Canister), and to be released to the engine air intake manifold at a later time. The system is controlled by the engine control module (ECM) via a vent control and purge control valve. The vent control valve, when open, lets fresh air into the charcoal canister, purging the canister of fuel vapors and directing them to the intake manifold to be burned along side the engine's regular air/fuel mixture.
The ECM performs fuel vapor leak tests to ensure the EVAP system is functioning properly. The test occurs while your vehicle is being driven and also when your vehicle is parked (engine off). During the vapor leak test, the ECM commands the vent and purge control valves to close. The ECM then monitors the system for pressure/vacuum loss. If the ECM detects a leak, it records a trouble code. Large EVAP vapor leaks are recorded as trouble code P0455, while small leaks are recorded as trouble code P0456. Here are possible engine faults which may cause a P0455 trouble code to be recorded by the ECM
Fuel cap left open or not closed tight enough
Defective Fuel cap (cracked) or damaged Fuel cap seal
Disconnected or cracked EVAP hose(s)
Defective or stuck-open vent control valve/solenoid
Defective or stuck open purge valve/solenoid
Damaged or cracked charcoal EVAP canister
Might have adjusted itself while trying to suck in all that air and that was the only thing I tightened up.