I just recently bought a 1997 Cadillac Deville Concours off of ebay. It is a beautiful car and i love it. Only one problem...the service engine light came on. I went to auto zone to get this checked out and they said that it is my o2 sensors. How serious of a problem is this? I know it regulates my gas/air mixture and I wont get as good gas mileage. This is my first cadillac with a northstar in it and I DONT want to mess that up! So please respond and tell me is this a serious problem? and where can I get the cheapest o2 sensors? thanks!