The P0440 code simply mean the system detects a leak....often referred to the "gas cap code" because an incorrectly installed gas cap will cause the code to set. Any leak in the system can cause the code to set, so any split/cut/disconnected hose, a leaking sending unit "O" ring, stuck open canister vent control valve, bad evap pressure sensor, leaking fuel filler tube, bad gas cap vent, etc., will be a possible cause. Sometimes the liner of the evap canister will fail and small bits of charcoal can get sucked into the system and create problems. Gas cap failure is relatively rare; vent control valve failure is pretty common. It's also important to note there were two different style vent control valves and, subsequently, two different canisters, so you must get the correct vent control valve to match the canister if replacing it. The old style valve mounted flush against the canister and the canister had a groove on the back where the "O" seal sits in. The newere style valve has an extension with the "O" seal on it that slides into the back of the canister. Also, if you use your Hbody for hauling a boat and use it to lower it into the water, water can enter into the canister. Nissan came out with an updated water seperator system to address this issue; there's a TSB on it. If the leak is hard to find, then a smoke machine is a mechanic's best friend. It'll hook up to the EVAP test port in the engine compartment. The vent control valve is closed via a scan tool and smoke is sent through the EVAP system to help pinpoint the leak. This is an expensive piece of equipment so it may be tough to find a shop that has one outside of a dealership.