I just got it out of the shop where it had an o2 sensor replaced and the mechanic suggested that I replace the spark plugs because it was still running rough. I replaced them and now the car is running worse than ever. It is now backfiring and feels like it is being choked out when I try to accelerate. Any ideas?

 

Volvos use strangely small spark plug gaps. I think that might be something to discuss. What gap do you have in there now? And what plugs? Plugs that are difficult to fire could also "run worse" than old plugs.

Once we get past that, the experts will have many questions I am sure.