Q: why am i haveing so much trouble with gas cap and "service the enjine" light? on 2002 Saturn Vue

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I have taken my vehicle which has 67,000 miles on it, twice to a certified Saturn location for the "service the enjine" light on. The first time they did a diagnoic test and said I didn't have my gas cap on tight enough. The second time they told me the gas cap would have to be replaced and they did, but as I drove home I noticed a strange vibaration as I was going up a hill that had not been there before, almost like it wasn't getting enough gas. Now I am taking it back again (the light is on again) and I don't think they have a clue what is wrong with it. Anyone else have this problem?
(1) Answer
We had replaced our gas cap and this kept happening and then my husband took the gas cap back and bought on that was made specifically for the Vue. It cost twice as much, but we havent had the problem since.