What exactly are the functions of these nox sensors? anyone?

My 94 850 Turbo check engine light came on soon after the service light. Almost each 5K service costed me around $400-$500 after the warranty had expired (I almost gave up on this car). I didn't feel any different in driving (ie car performance while the check engine light was on).

Since I replaced the turbo ( > 17k miles ago) the check engine light has stayed put. I am still ??? if the check engine light/nox sensor has anything to do with Turbo was heading south. Or could it be that the check engine light was disconnected by accident?

I was driving a S70 coupe loaner for a week. The very first morning, the check engine light popped up, the car had only less than 2K miles on it. I called the Service Supervisor. He asked if I'd filled gas with the engine running. I didn't. "Don't worry about it," he said. I felt uneasy to drive car with any warning light on, even it was a loaner car. Whereas my siblings choose to igore their cars check engine lights, nothing has happened to thier cars so far (Madza 626 and Madza truck).

gl