My 2001 Suburban always sets the P0440 code when it does the evap test. It passes the test, but triggers the CEL. all tests show complete at this point, but when I reset the CEL, they all have to run again. This can take a few days to a month or more, but every time the evap test runs, it completes but throws the code P0440. I have replaced the vent solenoid, but that didn't fix it. I have tested the evap solenoid, and it tests good. On a few occasions on a road trip, the CEL came on. I checked it while I was driving, and it was the P0440, so I kept on going. After an hour or 2, I stopped for fuel, and the tank had such a vacuum I couldn't hardly get the gas cap off!
Since the vehicle is now 25 years old, I don't have to get the emissions test anymore, but I'd still like to find out the issue. It still gets 17 - 18 mpg on the highway, and runs great It's only got 355k on it, so I want to keep it a while!