:flag: Hi Guys ,Thanks for all the great info and help everyone brings to the boards. I have a 2000 M5 with 54K miles now and was wondering when you should replace your oxygen sensors. My gas mileage seems to be a little lower lately ( avg. around 11-14 combined city and frwy. Thats with normal driving) I see the adds in the magazines for replacing your oxy sensor for better gas mileage. Should this be part of your 60K service or is it a waste of money? If so, WWhich of the oxy sensors should be replace? The pre cat or post cat sensors or all. Thanks for any info on this.