question, the 02 sensors, upstream, are there to moniter oxygen in exhaust so ecu knows if it needs to lean or richen fuel mixture, the downstream check on cat to see its doing its job or so ive read? ok, my exhaust is factory setup off mountaineer to the end of daul cats. the 02s are on both sides, one in front of pre cat...one after them...none after the daul cats..so nothing is checking the cats, just the pre cats. she runs pretty decent, idles nice, starts nice...has trouble once I'm at about 40 and in 3rd, then she seems to hesitate. going to look at the plugs today...even tho its maybe 16 degrees out side..lol