Ok so...first off...id never hack up a truck like this. I was able to find 3 o2 sensor plugs. On the left side of the truck i found a flat 4 wire plug. Bought a flat four wire direct fit o2 senso . When i plug it in i do see B1S1 come alive on my data stream. Unplug it and it's steady 450 mv. The other 2 wires are both square plug. So i went and also bought a square plug direct fit upstream sensor. I figured one of them must be upstream. Nope. I plug then in and get b1s2 and b2s2 readings in the data. I cant even find a 4th plug anywhere in the truck. Nowhere the harness is cut.. not stuffed on top of the tranny... there is just no 4th plug i can find. Anyone know if there is supposed to be one and where it may come out of the harness? So i took the passenger inner fender out to fix that cluster f of wires in there and pull that heat shield out (wasnt mounted and rattling around) no plug there either btw.. but i cant drive it just yet to cycle the emmisions. However i got 0 volts on the 02 sensor at idle. Is it possible im to far away from the block? The pipe comes out of the shorty header and crosses over to the passenger side then turns 90 and heads back. The sensor is about 6 inches or so after the bend. It does run up to .9 volts on start but then falls right down to 0-.1. Giving it throttle makes no difference. Also there is no trim after idling and simulating load and putting it in gear and giving it gas while i hold the brake. At this point im wondering if its mandatory both s sensors are connected and where that 4th plug is.