I'm getting P0136 bank 1 sensor 2 circuit malfunction from my Toyota Camry 4 cylinder, which from past experience means I need to replace it. I've replaced the O2 sensor in the exhaust manifold before. However, is sensor 2 the sensor in the exhaust manifold, or is it the one underneath the car?
Bank 1 Sensor 2 should be the sensor downstream of the catalytic converter. Bank 1 Sensor 1 would be upstream of the catalytic converter. With a 4 cylinder, everything should be Bank 1 (No Bank 2).
So, Bank 1 Sensor 2 should be the one under the car. The one on/at the exhaust manifold should be Bank 1 Sensor 1.
You might eyeball the wiring and / or check it with a multimeter, it's possible you've just got a broken wire or a bad ground. Here's a video. You can also use a scan tool and see what the powertrain controller is seeing on a vehicle with OBDII.