I have been getting this code for the last few fillups and it would go away in a few days. So I decided to get to the bottom of it.

Here's a pretty good explaination of the trouble code:
P0440 - Evaporative Emission Control System Malfunction - on OBD-Codes.com

Here's my take on the whole thing. Typically this code is caused by nothing less than a dirty or loose gas cap. Dirt can collect in the pressure vent valve and/or on the o-ring seal on the cap. Leaks at either of these areas can trigger the code. Other things that may cause this code include over filling your fuel tank which may allow a small amount of fuel into the vent system, causing a slight obstruction that will clear itself over the course of a day or two.

Now for my case.. Well, the pictures speak for themselves. Looks like it's time for a new o-ring if they're available. If not, a new cap is in order.

1st, the o-ring location:


2nd, the o-ring out in the open:


And when I say o-ring... Well, it isn't a typical o-ring with a solid circular cross-section. It has more of a U on it's side cross-section that should form a double lip seal on the inside of the filler neck.

 

Kyle,

The code may be caused by your gas cap sealing ring, but it also may not. I've had this code before and a new cap did not resolve the problem. The EVAP system has basically 2 "electrical" points of input, there are 2 EVAP sensors that provide input to the ECM. There is a sensor on the intake (I replaced that one and it did not resolve my problem), the other sensor was tucked above my spare tire under the truck. This sensor is the charcoal canister vent solenoid. When the ECM signals this solenoid to open, the solenoid opens and allows fresh air into the system which in turn allows fuel vapors to be sucked into the intake to be burned off (emission controls). Replacing this little vent valve (approx. $35.00 at the dealer) SOLVED my problem. The vent could also be plugged. When I finally found this valve and took it out, there is a fibrous filter in there that could get plugged. Has your ride been through any deep mud or such lately or ever. Crud could also build up in there over time.

 

I wasn't stating that I had any further problems. My problem was the gas cap. This was an informational post, not a I have a problem I can't figure out.

The link I provided does state the other areas that it can be. In my experience, the gas cap causes 75% of these problems. The other 25% can be attributed to either a faulty solenoid (there are two) or a leaking line somewhere in the evap system.

 

I found this forum and hope maybe you've seen this problem. About a week ago my SES light came on (2003 Chevy Blazer). I grabbed the manual (I am not a mechanic), found the appropriate section and it said to reinstall the gas cap according to instructions. I did this and after a few trips it went off, but has now come back on. Now the weird part....I tried to put gas in myBlazer and the pump kept shutting down like it was full of fuel. It took about 10 minutes to put in 5 gallons. Any thoughts on why this is happening? Thanks in advance.