Volvo: 96 Volvo 850 GLT check engine light, volvo 850 glt, volvo 850

Question
I started my car today and I noticed the check engine light was on.  The manual says that this means there is a malfuntion in the exhaust system which makes me think it might be the O2 sensor.  What does the check engine light typically mean and how much/how long will it take to fix?

Answer
Marcin,
Yes if the check engine light is on there is a stored fault in the system. That might be an emission failure, some sensor on the engine, a runability issue. No way to tell until the codes are read. How long it takes or at what cost is dependent on what fault is found.