1989 Pontiac Sunbird Excessive fuel useage

1989 Pontiac Sunbird 4 cyl Automatic

Hi, my son just told me that his 1989 Pontiac Sunbird LE 2.0 Liter with A/C, TBI, SOHC, VIN Code "K" used a half tank of gas in two days. He only went approximately 50 miles. He also said that it is running good but the check engine light is coming on intermittently after he has been driving for a while, doesn't come on when the car is cold and still runs fine when the light does come on. What would cause such an increase in gas useage if it is running good?

Thank you.