Oh, the notorious code P0440 -- every mechanic worth his salt knows by heart exactly what it means. Back in the mid-1990s, the government mandated use of a comprehensive diagnostic and emissions testing system known as OBD-II, which monitored dozens of things older systems didn't. Unfortunately, it took a little time for manufacturers to learn to build cars that held mechanically to OBD-II's electronic standards. And such is the case here.
This code is something of an in-joke among many mechanics, particularly those experiences with late-1990s and early-2000s automobiles. One major source of automotive emissions back in the day was simple evaporation of fuel from the vehicle's gas tank, particularly on hot days. Gasoline is very unstable, especially when warm; this unwanted evaporation not only cost money in terms of lost fuel, it sent unburned hydrocarbons into the air for no reason at all other than a poorly sealed fuel system. One of OBD-II's testing parameters involves vacuum-testing the fuel system, starting at the vent hoses leading to the charcoal canister filter. If the system detects a vacuum loss in the fuel system, it will assume there's an air leak somewhere in it. When the car stops, fuel will evaporate out of that leak and into the air.
Code P0440 is probably the most common fault code out there, because it often comes down to user error. To wit: It usually means you didn't close your gas cap all the way. Or, if you did, the seals and vent mechanism on the gas cap may have malfunctioned, and you need a new cap. Newer cars are better about gas tank sealing, but this kind of thing isn't uncommon for cars produced in the early days of OBD-II diagnostics. Of course, it could also indicate a legitimate leak in the hoses going to the charcoal canister under your hood -- but most often, it's the gas cap.
The quick test is to listen for a quick hiss of air when you open the gas cap immediately after shutting the engine off. There should be a pronounced vacuum in the tank; if not, you likely have a leaking gas cap, or it's not closed all the way. Once you've tracked down the source of the fault, you can use a generic OBD-II scan tool, available for loan at most auto parts stores, to clear the stored trouble code.
