If you look closely at the listing of the code from the document above, you can see that it will only throw the code when your fuel level is less than 85%, and greater than 15%; that would explain why you get it right when/before you reach 3/4 of a tank, giving room for error.
Where did you get the definition of the code P0440 as tank vent sensor? If you are using an "interpretive" code reader or a cross-reference for codes, it may list that. But it is best to look at the GM definitions for codes (the link to the document above), as they may vary slightly from the code reader. This code appears to not be a sensor code, but a problem code.
The difference in code is because yours is a GMT800, and mine is a GMT900, but if you decipher the information in the GM codes, we are getting essentially the same code. P0496 doesn't exist for you, while P0440 does not exist for me, but it is the same problem!
Mine hasn't done it again (it's only been two days). I was wondering if parking on a slope would affect this. I had just reached 90% of my fuel tank (GMT900 criteria for this code is 90%, not 85%), and I was parked on the side of a street with a slope, so the driver's side (fuel tank side) was lower than the passenger side. I wonder if the fuel, being so full, somehow forced the purge system to operate when it was not supposed to. I'm guessing that is why the >10% or <90% criteria is in this code. Do you park on a slope?
I would look again at your sensor, and possible the purge valve itself. There usually is a separate code for sensor malfunction, so if this is the only code you are getting, you can bet the valve is probably bad. However, your history of cleaning it to temporarily fix the problem lends itself to question the sensor. Depending on price, you could do this first and then do the purge valve. But if the purge valve isn't too expensive, may as well do that while you are at it.
I'm going to wait mine out before I do anything to see if it throws the code again. I'm hoping it was just a one-time deal due to fuel level conditions. I wouldn't expect any problems with my truck being newer (2008), but you never know. This must be why emissions warranty is 24,000 or two years, depending upon the state you live in.
I hope this helps.
