I have a 1996 850 non turbo with 151,000 mi and a check engine light that will only stay off for around 200-250 miles after reset. I have replaced the O2 sensors and had the vacuum lines checked by a, I believe, reputable mechanic. First they told me that the problem was that I used a Bosch direct fit 15097 O2 sensor in the front. They then advised me to, at a minimum, replace the rear O2 sensor as well. I had them do that and they used a "geniune" Volvo sensor from the dealer for that fix (ouch!). After that failed, they told me that compression was low in two cylinders (2 and 4). The car is burning some oil, but still seems to run strong. I have read, and been told, that I need to reset the drive which seems tedious at best, but if it is necessary, I'll do it. I love this car. Any good links to this process, or am I going in the wrong direction? My emissions inspection expires this coming Monday.