Since the #2 sensors are further down the exhaust stream than the #1s, they will take longer to heat up. They should be ready within a few minutes of engine on however and will start varying voltage even at idle if they are good within at least 5 minutes. If the voltage does not change at all the ECU will set the code you are getting.
Under the hood on the passenger side are the two connectors for the rears. Green and Brown with 4 wires each. The black wire that is segregated from the rest is the sense line or + out for the sensor and one over from that is the ground return. The other two wires...gray I think..are for the heaters.
You could check the connections here for corrosion and then follow the wires under the car and look for damage. The heater circuits are monitored by the ECU for continuity and current draw when on so you should get an O2 heater code if something is up with that. Do you have any mods to the exhaust system? If the engine idles high, it might be a vacuum leak. A bad or dirty sensor can cause this also.
Quick and easy..........Try swapping the green for the brown temporarily and see if the code changes to the other side....P0156 i think. Someone here with more experience on fuel trim data might be able to help you sort it by looking at those values if you can't find anything else wrong.
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