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Question: The check engine light went on then off?

A couple months ago, my check engine light went on as I left the gas station. I was on my way to see family, including an uncle who is a retired mechanic. He scanned the light, and it said emissions. He tightened the gas cap, and said it should be fine. When driving home, it came back on. With a call placed, he said that it may be the seal on the cap, he'd try to find a replacement. But it would be fine to drive. So, after, when I tried to fill the tank, it would stop while fueling. Recently, I started going to a different station and haven't had a problem filling at all, then today, the light went off. What's up? (Mechanic uncle lives far, so he's doing the best he can, but it's hard to get up there, and I want to be sure I don't get ripped off).