Volvo: black smoke out of the exhaust, fuel mixture, o2 sensor

Question
I'm looking at buying a 1999 c70, the price is very good, so I'm worried. Of course the check engine light is on, but the idling is rough and the exhaust is black. They say it just needs an O2 sensor and they will put that on tomorrow. When I go back, if the problems have cleared up, do you think they may be all it was? Would that cause these problems? It has 116,000 lbs and is a convertible.

Answer
Curt,
Yes it is possible that the O2 sensor is the cause of the black smoke. The O2 sensor along with other sensor helps to set the fuel mixture. I would take the car to a shop that knows Volvo and have them give the car an inspection so you know how the rest of the car looks.