p0440 is a code that the computer sets when it sees that your EVAP system is not working correctly. What is an EVAP system? Well all it really is is a way for the fumes that are generated from your fuel sloshing around and expanding on hot days to be stored until they can be sucked into the engine and burned.
In order to do this car makers put a charcoal canister (this part just stores all of these fumes) in the engine compartment and install hoses between this canister and the fuel tank. When there are too many fumes to fit into the gas tank they all travel through these hoses to the charcoal canister and they are kept there until the computer determines that they can be used in the engine and it opens a valve to let them be sucked into the engine.
So why the code? Well, if one of these hoses or the valve that lets the fumes go into the engine malfunctions then there would possibly be gas fumes going into the air. This is not good for anyone so when the computer sees this happening it will set this code.
The first thing to try is re-installing your fuel cap as this can cause this code and is an easy fix. If you suspect the cap might be bad, then you might want to just replace it. You can also check for any loose or damaged hoses under your hood. They can cause this type of code.
If that does not fix it then I recommend taking your car to a good repair shop. They will have what is called a "smoke machine". This machine puts smoke into the whole system and then they can see exactly where it is leaking. There is no other good way of checking for leaks in this system. It will save anyone lots of time and money to just take your car somewhere that has the capability to perform this test.