Saturn SC-1 1995

I just about 2 weeks ago got the worst problem in my life a 1995 Saturn sc-1 5-speed with sunroof a nice looking red body motor is the SOHC and runs ok but 2 weeks ago it ran great now it will run hot if you sit still for about 20 mins and the major decline in horsepower and gas mileage is bad and the Check engine light came on last night coming home for work (was not running hot) so when I got home I jumped the A and B terminals in the ALDL and code 12 was the only code the flashed but when I jumped the A and B terminals and turned the key on the eng fan came on but without the jumper the fan don't work at all when I first got my saturn the gas mileage was great would fillup on Friday and drive to and from work all week(about 50 miles per day)but now I fillup on Friday and on Monday or Tuesday the gas tank is empty don't have any gas leaking all the line are good this pretty little red head is driving me crazy any ideals would be great thanks for you time