Hello All,

My independent mechanic just replaced the two O2 sensors on my 1996 850 when the "Check Engine" light came on. He said his shop is now following the practice of dealerships of replacing both sensors at the same time because "It has come back to bite them when they didn't". I assume he meant that a week later the other sensor failed. At $250 a pop plus the additional labor the bill was in excess of $700. I'm questioning whether the "economics" of this decision in in my best interest. Putting that issue aside, I know the O2 sensors will fail again, so I would like to replace them myself in the future. Can anyone telll me the following:

1) Once the CEL comes on, is all I need to confirm it is the O2 sensor os to use an OBI II Scanner? If so, where can I get one, and what should I expect to pay? Also, what are the code(s) that confirm the O2 sensors are bad?

2) How difficult is it to remove these sensors? I know one screws in the front part of the exhaust (the catalytic converter?) and the other in the rear portion of the exhaust system. I think I need a special "open" wrench that iPd sells to loosen these sensors. Is it very difficult to gain access to these sensors, and is it a nightmare to get them off because they "fuse" to the metal after a while because of the high temperature they are exposed to? Is there any other type of tool I need to do this job?

3)Is there any web site with detailed instructions and pictures to show how this replacement of these sensors is performed.

Thanks in advance to all who repond. Your assistance helps to reduce the costs of maintenance on these cars.

Hugh