What o2 sensors did you eventually fit? I looked at the price you said on your earlier question the other week and my sensors from Volvo cost £145GBP each which now is about $290 I think you stated about $165. You have the same problem that i had. OK, so i know each car acts differently, some may like the cheaper sensors but mine didn`t. The code also did not give any idea, it did at first when it went wrong but the cheaper spliced sensors were fitted at a local ordinary garage £50 each ,but i still had rough running and missing of cylinders with black smoke and 12 to the gallon but the o2 sensor code had gone. I suppose the code you have is still true, missing on cylinders, but does not point to the cause. It took me ages and another £350 to get it right by the proper Volvo dealer who put the sensors in. I hope for your sake it is the grounding earth wires, at least it will only cost you 2 star washers more!! Best Regards