07 yukon code p0455 not clearing

Hi.
First of all thanks for accepting me as a member of this awesome forum.

Now, I just bought a 07 Yukon which seems to be in a great condition. The check engine light just popped up so I went to a mechanic and he said the code is p0455.

He did a smoke test and showed me that smoke is coming out of the rear of the vehicle at the gas tank area and the car needs the vent valve solenoid changed, but it's only sold at dealer.

So u went to dealer and got this part and decided to change it myself which was quite easy and fun.

I went back to clear the light, then a week later the light came back on, so I went back to him thinking maybe I didn't do it right, so he did another smoke test and sure enough we see smoke coming out of the rear again, so I looked under the car and saw it was coming out of the end of the vent valve assembly which I think is a small air filter. Is that normal? Or is the part defective?

Maybe my problem is something else that I missed?
If anybody could suggest to me what I should do I will really appreciate.

Thanks
JF858

If the vent valve was in the open position this would be normal, I would google that code to get some definitive info, a bi-directional scanner can command those solenoids on and off for a better diagnosis. https://www.youtube.com/watch?v=E6yLLqAxhcs

I got a new OEM gas cap, cleared the code and all is fine now. Thanks all for your help!