Dave,
Unlike your 850, the s 80 especially 99, Its a dealer issue. If your dealer charges 200 bucks to find out the code then they are thieves! it shouldnt be more than a hundred if you are out of warranty! If its been on for months I would say its likely a o2 sensor which you have 4 of. If you got the money take it in to see what the code is, you dont have to fix it, if not then wait till your car runs bad or maybe dies. I know that sounds bad but if it was a serious engine problem you would have known by now. Im guessing a oxygen sensor, which will just affect you gas mileage and perhaps a bad odor from your car.