it is the sensor that is just off the exhaust mainfold ussualy under the spark plugs
The bank 1, sensor 3, O2 sensor would be the O2 sensor post cat.
Is this bank 2 sensor 1 or sensor 2?
That would be the O2 sensor after the catalytic converter.
usually bank 1 is the bank with # 1 cylinder in it and sensor # 1 is the front o2 sensor(usually in the manifold). You will need a O2 sensor socket or a 22mm wrench or deep socket.
Assuming this is a V-6, bank 1 sensor 1 would be the first O2 sensor in the exhaust on the drivers side of the engine.
That is the O2 sensor behind the catalytic converter.
Passenger side is bank one on a LHS.
That would be the O2 sensor located after the catalytic converter.
You need to change the upstream sensor
the O2 Sensor Should be on top of your exhaust manifold
bank one is where the number 1 cylinder is sensor 1 is before the cat and sensor 2 is after the cat
O2 sensor CKT No Activity (Bank 1 Sensor 1)