oldchev

I will end this post with my fix to this problem. It was the canister vent valve located at the rear of the vehicle. I took the valve off and bench tested it with a vacuum source. Drawing through the vent tube side (to gas tank) of the valve I engaged the solenoid a dozen or more times. First only clicking of the solenoid but was still able to draw a reduced amount of air through when energized (the leak). After working it several times and some silicon spray the valve closed on energizing and sealed. I reinstalled the valve and drove it about 6 miles and the MIL light went off. As I mentioned earlier the valve was very dirty and dusty when first removed. I assumed a grain of dust prevented full movement of the solenoid valve to close. P0455 large leak: although gas cap was the message (probably the most common) it was the valve that wouldn't seat. I will replace the valve with a new one, they're cheap enough. -oldchev

The 2004 Denali has now set the P0455 code again. I have read numerous blogs and forums relating to this and of course, they first point to the gas cap, which I am sure it is not. The purge valve, normally closed creates the vacuum for the test (besides drawing vapors into the intake) then closes (no power) for the test. The vent valve (in back) normally open (no power) closes to seal for the test when activated. I intend to test these valves on bench for function. Without dropping the tank, I can then only suspect a hose, or the pressure sensor. Next chance will pour over this again, do the tests on the solenoid valves, check all visable hoses. I may have to indulge making my own smoke tester and see how that goes. I know the factory techs probably have a shortcut or probable cause for this so wondering if anyone has this insight. I don't give up easily and am relentlessly a DIY guy. I have the manuals for this vehicle and I am using the Innova 3130c for code retrieval and erase. The $$$K scanner I do not have. Suggestions?

My 2004 Yukon Denali (w/6.0) has the tighten gas cap message irregularly. I bought used and was OK for 4 months them the MIL light came on. I have a scanner and the P0455 code registered. The gas cap was not original so bought the NAPA Stand cap and put it on. Not the fix. I then used silicon spray on the first cap to allow smoother seating and made sure tight, was trouble free for a couple days then P0455 set again.I used the scanner to erase the code and I retightened it and was good for a couple days then the tighten gas cap message appeared. No code. So I relented and bought the correct AC Delco cap. Same message. I then took off he vent valve solenoid assembly from evap tank and cleaned the filter and dust from it and check the solenoid for function w/12 volts. happily I reinstalled it and it just told me to “tighten gas cap”. Hoses and lines looked good at the evap canister etc. and fuel tank fill lines. Next: ??? Maybe the purge valve solenoid up front? Have a smoke test? Funny it set the codes real fast at first, then just the message.