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I replaced all 4 HO2S on my 2004 Aviator from underneath. The Sensor 2 sensors were pretty easy to change out. The tougher of the 4 sensors were the pre-cat sensors.
Based on the P0136 code, be sure the inspect the wiring when replacing the sensor. And not that you asked, but B1S2 would be the sensor after the catalytic converter on the right side of the truck (passenger side).