My check engine light has been coming on/off intermitently. My mechanic get codes indicating a front o2 sensor. The rear has been repklaced twice. A $100 scan at the dealership indicates a front o2 and possibly a MAF problem. My mechanic whom I trust thinks it is actually the catalytic converter. On another possibly related side note, when the temps got real low this winter, I got this load almost gurgling sound from the engine on the passanger side when I would first be driving. Once I had been driving for about 5-10 minutes it would eventually stop. It did not occur when the car was stopped with engine idling. Could there be some sort of problem with the air pump system thereby creating a domino effect and damaging the o2 sensors as well as the catalytic converterr? Incidentally, the rough runs great but a little rougher than our other 99' XC.