Been getting a P0441 intermittently for a year or two. Would stay off for months, come back on for months... etc. Started seeing P0440 recently.

Recently, I looked under the hood and found a loose hose connection at the intake, coming from the purge solenoid. Tightened this, cleared the light, took it to emissions inspection a few days later where it PASSED with no codes.

However, 8 hours later, P0440 is back.

Any ideas -- considering the car passed emissions, I'm stumped.

 

There's a lot more to the evap system than that solenoid connection. The fuel filler cap, the leak detection pump, the cannister, the connection hoses, the solenoid, etc, etc.
If you want to fix it, you'll need to check the whole system.

 

I realize there's a lot to it. I guess what I'm getting at is, I did a visual inspection underneath the hood, and the car passed emissions. If it passed emissions, then surely this would narrow it down? It would seem that it wouldn't pass if I had a leak, or a complete failure. Perhaps I have a part that is intermittently failing, a diagnostic issue (LDP?), or its related to a changing variable such as my fuel tank level? Tried the gas cap, aftermarket cap did not prevent code.

Looking for guidance. I have the dealer manual for this car, but in most cases, it cops out and refers to the $8k diagnostic device. Didn't even see a P0440 reference in the guide, but saw a P0441.. which I'm not getting right now.

 

The P0440 code is a generic code:

"P0440 Evaporative Emission Control System Malfunction"

It obviously doesn't narrow it down much. If you have a vacuum pump you might be able to check the function of the solenoid. Other than that, you are pretty much confined to visual inspection of the various parts of the system. That aftermarket gas cap should not be beyond suspicion either.

 

OEM gas cap is back on.

Does anyone have an idea on what this is USUALLY caused by on a vehicle like this?