When I got the misfire on two cylinder reading after the check engine light came on, the mechanic guessed it was the oxygen sensor. He reset the code and it has not come on again. I ordered a new rear sensor but with the light still off it is easy to forget to change it out. I just started with the rear sensor because it is easier to see and change. Seifeldt Volvo said on phone that they usually change both out at the same time.