I recently took my 850T to the dealer for check engine light. The said, "Fault traced ECM and found multiple misfire codes, check plugs, ok. Note: Monitored front O2 sensor and found it slow to respond, recommend front O2 sensor, EST. 391.60 plus Tax."

So they reset my service light and it came back on. This time, I went to Bay 13 and learned how to read the codes myself. The only code was 425, which sounds like it alludes to the rear sensor going bad.

How can I tell which one I need to change? Should I change both, I can change both myself for less than the dealer charges me for one. Any thoughs/comments would be appreciated.

you can check the functioning of the O2 sensors yourself.
I wouldn't suspect multiple misfires to be caused by a faulty or two faulty o2 sensors. I think the rear one just monitors the exhaust to ensure the cat is working ok.

how many miles? check the plug wires the cap and rotor, vacuum leaks.

Now some one on this board have save some big $$$$$$$$$$$$$$$$

by taking the O2 out and using a blow torch and burn out all the bad carbon deposit and plug it back it work fine ..

sensor go bad because carbon deposit on the surface if you remove the carbon it should be fine...

sound like cheap Charle but hey if it work why give your $$$$$$$ away.. you can by a can of blow torch at any store for $5 and some beer you done....

i know that you have been to the dealer and i'm not trying to second guess them. but....sounds to me like you could have a vacuum leak. it's very seldom to have both sensors go bad, esp. the rear one. i'd check all the vacuum lines first. i had the same experience. i found a bad rubber elbow by the intake manifold and a disconnected one on the mail intake hose (after the maf sensor, just under the turborcharger. bay13 could help you locate the more common places where leaks could occur.
good luck.

I had the same problem with my front sensor. I bought a Universal Bosch O2 sensor and replaced it myself. It's a little tricky to install, you have to splice wires, and there's not a lot of room under the car. It's easier to unplug the sensor. I couldn't get the plug connector out on mine. Anyway, the universal sensor was only $100 Canadian as opposed to $250 for the “real” sensor. It works just fine. I can't remember the part number off hand, but the parts store cross referrenced it to the non-universal one.

I have been looking for a Canadian site to buy the universal sensor. So far i have only been able to find it in the states for about $100 USD and they would not ship to Canada. Can you give me the web site or phone number of the place you got yours.
Thanks
Jeff
1994 854 T5
1994 855 T5

Try to put in a bottle of Fuel Injector Cleaner and see if it clears the code. This works for me when I have a bunch of different codes like misfires, O2 sensor etc. Costs me $10CAD to fix the problem.

The Bosch web site can do a part lookup and give the recommended
Bosch O2 sensor part numbers.