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engine code P0440 on 2002 GMC Yukon

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Light is on, won't go off even after reset. Changed gas cap.....thoughts on that to do next? Seems to be running just fine
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There are typically three common problems that set of a code PO440: 1) would be the fuel filler cap 2) would be the evap canister vent solenoid/valve 3) evap cannister purge solenoid/valve.

If you have already changed the gas cap then assuming it was a good quality one then we should be able to rule that one out.
If you drive on country or roads with lots of dust then the evap canister would be the next place to look. I have several customers that drive dusty roads and we've almost gone to changing them with the fuel filter change every fifteen thousand miles. It could also be a few other things like a fuel filler neck problem. I would start with the canister and go from there.