Downstream O2 sensor all the way, if you look at the wiring leading up from it it will cross over the passenger side axle, allot of times they meet it owns the wires and causes a short. The Fuel Pump, Primary PCM power, and O2 sensors are all on the same circuit. I had the exact problem, new wires on the O2 sensors and a zip tie to hold them out of the way and it made it all better.