I have a 2005 Tahoe. I filled gas tank up, drove 30 miles, parked it, and when I cranked it back up the check engine light came on and the warning to tighten my gas cap came on. So I did, and sure enough I had not tightened the gas cap. Drove 30 miles back home and light was still on so drove it a couple more days and it never went off. So took it to Auto Zone and they hooked it up and said the code stated it was the gas cap. So bought new gas cap from them. Put it on, light never went off. Took it to mechanic and he said same thing, code was showing the gas cap was the problem. So went and ordered one from the dealership. Put it on, light stayed on. Took back to mechanic, he reset computer and put a ring in the cap just to make sure it was sealing good. Drove for a few days then light came back on. Mechanic guy said it would not hurt the vehicle to drive it, so I did. The light would go off at times and come back on at times but still is on now. Then time for inspection so I had to get it fixed, won't pass because darned light. So it is now at the mechanic shop and the code they get is still that it is the gas cap. They ran the smoke in it and found no leaks at all. They have checked everything and are finding nothing wrong. Man said he has never ran into an issue such as this before and is still trying to figure out the problem. Does anyone have a clue what else this could be?
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Take it to a better mechanic. There is no code that says "The gas cap is bad". It could be a possible fix for a code or two but obviously not yours. Autozone parts people are not mechanics, the code never tells you what is wrong it only tells you where to start looking. Autozone"s code reader also only tells you what parts are needed based on probability. What is the code number? Just because that guy has a smoke machine it does not mean he knows how to use it. He must close the vent valve with a scanner or a 12v power source, did he? I have seen Evap codes set for something as simple as spider webs in the vent line. Someone must take the code number then print out the trouble tree and run it down. Just because a guy has a tool box and tools does not mean he can diagnose problems. There are less guys that truly know how the systems in a car work (truly work) then you can imagine.
ASE Master Tech, L1 Advance Engine Performance certified
I will not answer repair questions by PM, if you would like my input you can PM me and ask me to reply to your public thread.
My wife's 2005 suburban has this same issue. I also occasionally get a vacuum leak warning too, but I was told that it comes from a bad seal from the gas cap. I ordered a new cap as well, but it hasn't come in yet. I'm guessing it probably won't fix it though......
without more info can't help much but im assuming you're getting the gross leak/no flow or large leak codes if people are telling you to get a gas cap, which honestly i have only replaced about 2 gas caps compared to countless other leaking parts of the evap system.
post the code and get it smoked tested otherwise its a guessing game