I have a 96 that is giving a "lean fuel on bank 1" (or something like that) code. Since the truck has over 100K miles, I wanted to check the O2 sensors. The Haynes book talks about the O2 sensor mounted in the exhaust pipe near the starter. But PartsGeek showed a rear sensor (the one I described, I suppose) and a front sensor. Question is where is this front sensor. I one mounted in each header somewhere?