Long story, but I am in CO and I'm flying back to Chicago to pick up my '94 855 Turbo. Before the car went to Chicago, the check engine light was on, and I had the codes read, and they were:
1. O2 sensor
2. running rich and
3. knock sensor(?)

Not too worried about the 3rd, hoping it'll go away if the O2 sensor is replaced...I'm pretty sure the "running rich" will for sure. My Q is: I forgot if it was the front or rear O2 sensor. I want to order the part from Groton before I go home....which is the sensor that usually goes first...car has 92k and I can't check the codes from here. Not looking for a definitive response here, I know it COULD be either, but which one (front or rear) do you folks think it is that has gone bad? Maybe the other codes will give you mechanically inclined folks some idea if it's the front or rear...

Thanks,
-Nick

As the other respondant said, good idea to probably do both at once. The front "pre cat" sensor primarly sets the fuel mixture and the one behind the cat "sees" how well the cat did at cleaning up the exhaust. Don't fool around with after market sensors. Get Bosch and make sure it is the correct one for your engine. Check discountoxygensensors.com or Groton.

bl

Thanks for the responses...I'll do both. Groton looks cheapest...
-Nick

On my '95 the front went first, then rear went about a year later. Buy both, you're going to need them. I've been told by a Bosch expert the sensors typically have a 100k lifespan.