If the 15 min battery disconnect doesn't work, you may try to find out the specific sensor your mechanic installed. I just went thru a royal O2 sensor fiasco with a 96 850. "Generic" sensors are generally a no no. It needs to be Bosch and the right one for your engine. I found out (after more $$$ wasted than I want to admit) that a lot of sensor charts that mechanics and web sites selling sensors use list the wrong # for the 850. I got one from an on line sensor vendor, installed it and reset the code. The car contiuued to run rich and set codes. I took it to my regular mechanic who swapped sensors again ( now I'm in $300.+). Seemed fine for a week and again began to set codes and run like crap. I figured it couldn't be the sensor so I looked everywhere else and I imagine destroyed my cat in the two weeks I screwed with it. Took it to the mechanic of the guy I bought it from (shoulda done that first) and he found the wrong sensor installed. He showed me his updated chart that listed a different sensor entirely. He installed that one (even more $$$) and it's perfect. Back to 25 mpg. Live and learn. As the saying goes, "learn from other peoples mistakes...you won't live long enough to make them all yourself".

bl