M54 Engine Management Woes !
Many, many thanks for responding so swiftly. From memory, I believe that this engine has two O2 lambda sensors, one upstream and one downstream of the cat. Are you referring to the latter i.e. the post-catalytic sensor ?
Since it seems that you know your onions, is this sensor a) difficult to change, and b) are the cheaper brands (other than Bosch) worth fitting ? They would appear to be about 50% cheaper here in the UK !
You ask about fuel octane. Whenever possible, I try to use Shell V-Power which I think is rated about 98. I give her a good, long drive every weekend. Isn't that sad ?!!