I just replaced both of my o2 sensors on my 97 850. I bought the parts at FCPGroton (yes, they DO have the best prices). Both parts cost $245 w/S&H. I'm definitely no expert mechanic, but I do most of the basic maintenance on my cars. I was surprised at how easy the sensors were to install (just make sure you pry the red side clamp open on the connectors). Also, the sensors did not ship with the pigtail you have to use on the front sensor, but FCPGroton sent me one Priority Mail at no charge (thanks guys, if you read this). I had to take it to the dealership and have them reset the adaptation after I put the new ones in, but I still came out about $275 cheaper than allowing them to do the complete installation. Hope this helps. Good luck!