I'm working through an '95 850 O2 problem myself; CE comes and goes, currently gone. the buzz on the board is that having these tow codes occur together is an indication of something other than dual failures, more likely vacumn leaks or poor/dirty connections as a dual failure is reportedly rare. In my case I replaced the front O2 with a Walker unit about 6 months ago at 110K miles. I got the 521 code and have managed to clear it up right now by trying to access the connector end of the sensor, removing and reinstalling the airbox hoses, reconnecting harnesses and cleaning connections, all in trying to get to the bloody O2 connector. I bought one of the Bosch 15715 universals from Pep boys for $50 and was ready to reinstall a new sensor when the CE light went out. We'll see. It is frustrating to hunt these down. Access is difficult but it seems the O2 sensors themselves are reliable but the system is very sensitive to changes in voltage, hence keeping everything clean and tight is half the battle. Good luck.
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