Gas mileage problem

For some reason when I drive on the highway after about 40-60 miles I will get a check engine light and my mileage will be about 13 mpg. I can drive 60 miles in and around the city and get about 15 mpg and not get a check engine light also. I cannot get the check engine light checked because whenever I turn the engine off and turn it back on the light will be gone until I drive about 20-40 miles.

Does anyone have any ideas what could be causing this??? the only one I can think of is the o2 sensor which happens to be good and stuck in the exhaust pipe. This is really killing me whenever I have to make a long drive.....

check out this posting. seems to be the same thing with a happy ending.

http://www.automotiveforums.com/vbulletin/showthread.php?t=666067