Today I had my gear position sensor replaced. Something I was considering myself since have the complete transmission manual with step by step instructions. After watching some of the "tricks" of removal and being charged one hours labor I'm glad a "pro" did the job...so why do these things fail? Volvo can't keep them on the shelves! Taking it apart you basically see three "brushes" that make contact with a copper arc. When the gear shifter is moved, the cable attached moves these contacts back and forth. Depending on where they are parked, the transmission control module now knows what gear it should be in and controls the transmission electronically. After seven years of service, I was shocked at the amount of crud was inside the sensor. There is LOTS of black grease used to help the brushes move back and forth, but at the same time gumming up the contacts along with small shavings that the brushes have left behind. No wonder these things fail...in fact when looking at it, it's a wonder the sensor works at all. The master machanic say this is not unique to 850s but all Volvos starting around late 80s....Volvo really needs to address this problem and come up with a new design...or the manufacturer/supplier should back off on the grease....my advice to those who have not had a problem so far. Excercise the shifter on occasion and run the full range of gears...spreading the lubricant around in the sensor. Seven years of going from Park to Drive every day soon collects the lubricant in one area...just a theory but one I wanted to share.