Front Air Bag Sensor Control problems of the 1999 Hyundai Elantra

The "air bag" and "check engine" lights stays on all the time. Both driver and rear passenger windows will not work, both must be taped up. Car has had a "electrial burning" smell off and on for a year. The headlights goes bright while driving. Getting less than 20 mpg gas. Batteries frequently goes dead.