My "Service Engine Soon" light is on... again

My "Service Engine Soon" light is on again, the the OBD2 sensor gave me the code P0440. I had already had this problem and thought I had fixed it when I replaced the Vapor Canister. Any other thoughts? I was thinking I should replace the Fuel Filter but that is all I can think to do.

I had that code when my fuel tank vent hose connection wasn't seated properly, after replacing a fuel pump.

Also, Check the vacuum hose to the ball under the hood, and all other vacuum hoses. Any break in a hose along the fuel system line could cause it.
Also make sure you have a good functioning gas cap.