Check Gas Cap/P0455

[rottonj]

So I have had code P0420 Catalyst System Efficiency Below Threshold pop up a couple times since owning the car for the last 9 months, the car runs great and while it possibly needs a catalytic converter it isn't getting one right now. So I decided to do the anti fouler trick on the rear o2 sensor. , drilled it out doubled them up and reinstalled. First drive out I was treated to a P0455 Large Evac leak with a check gas cap on the DIC. I replaced the gas cap when I first got the car with new GM cap, so it should be good. I cleared the code and will be monitoring it. I don't know if it is coincidence the P0455 popped right up after the anti foulers are installed, or that is directly related. Anyone who has seen this or can offer any input it is welcome. Thanks in advance.

[michael redmon]

I had that code, changed gas cap, cleared code, never came back

[Scottydoggs]

check the vent line from the tank to filler. mine was rusted through and had holes i guess, i could never find my leak when i had that code. i got a tuner, deleted the evap 100%.

years later i had a gas leak and found the vent tube hanging under the car. i patched it up with rubber hose.

if you look up to the filler neck from under the car, you'll see a fat tube for the gas to fill, and the smaller line is the vent. mine rotted apart right under its lowest mounting bolt.

[rottonj]

Thank you. It just seems strange to me this happened immediately following installing the antifouler. I will check the line , as well as monitor it.

[montylololgram]

Maybe this is a little late, however I had this same "Check Gas Cap" issue, I replaced the cap and the light stayed on, I believe that was the same code, I chased a few different EVAP sensors, and then I found it, being the 'EVAP Vent Solenoid/Purge Vent Solenoid' (something along those lines) underneath the rear bumper, on the left hand side. Look under the car and straight up near the gas filler neck and you'll an odd, soda can sized plastic canister, I replaced it and cleared the code. Never came back this was on 10/02/2016 @ 192,141miles.