Jeep Liberty Sport 2003 Check Engine Light?

I have a Jeep Liberty Sport 2003 and this past couple of week's the check engine light comes on when I refuel. I took it to the dealership, who said they couldn't find anything wrong and reset the light, but I just refueled and the light came back on. I don't believe the gas cap is... show more I have a Jeep Liberty Sport 2003 and this past couple of week's the check engine light comes on when I refuel.

I took it to the dealership, who said they couldn't find anything wrong and reset the light, but I just refueled and the light came back on.

I don't believe the gas cap is loose.

Additionally, I appear to be getting around 50 miles less per full tank of gas.

Obviously, I don't want to keep taking the car to the dealership and paying them to reset the light. They tried to charge me $145 for that alone on Friday.

What could this be, and is there a definitive test they can do to find out?
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