code P0440

I have had a P0440 code popping up every so often for the last 6 months. I know it is a emissions issue but after doing many "searches" I think I'm more confused than when I started on what it could be. (lol) It's a 2002 Tahoe with the 5.3 engine and seems to run fine. The only clue that has happened a couple of times, (but not always) is when you remove the gas cap,which I have replaced, pressure is relieved from the tank as if it was not venting. Has anyone had this problem that could help me with a fix, so I don't have to change a lot of parts until I find the right one. Thanks

Mine had that code, plus P0171, P0300, and P0174. Also, at idle it would run a little rough and miss. Some of the easier suggestions were to change the fuel filter and clean the mass air flow sensor, did those, did't help. One thing all the codes suggested as a possible problem was a vacuum leak around the intake manifold. Long story short, I replaced the intake manifold gasket about 3 weeks ago, so far everything is running great.

Thanks for the reply. Did you happen to notice your fuel tank not venting? That seems to be the only symptom I notice as the engine runs and idles fine.

No, I didn't seem to have that problem.