Reading the fault codes is a start, but a start is all it is. This does not necessarily mean the O2 sensor is at fault, even though of course it could be. Incidentally the O2 sensor Bank 1 (B1S1) is the sensor before the catalyst on bank 1. Bank 2 would be the second bank on a V engine, but since your engine isn't a V engine there is only one bank. B1S2 is the O2 sensor after the catalyst.

As I said, the O2 sensor may be at fault, but then again it may not be. Fault codes do not tell you what's wrong. They tell you where the fault is being reported from. In this case the O2 sensor which could be at fault, or it's values could be out of range because something else is at fault. The engine could be running too rich or too lean because if a fault or even multiple faults. These would have an effect on the O2 sensors output signals, which could be telling the truth. You need to determine which it is. Just replacing the sensor is only a guess, and you could be replacing a perfectly good one.

Google fuel trim and how to check it. Also use a vacuum gauge to determine if you have any mechanical engine faults. If you do then you'll need to correct these first otherwise your fault code will keep coming back.

Once you clear the fault code you will also clear the fuel trim and readiness monitors, resetting them to 'Not Ready' or 'Incomplete'. They will not become ready again until the car has completed a drive cycle which could be up to four days. This is why the engine management lamp takes this long to come back on. If this is the case then it's good evidence that your problem is mechanical rather than electrical.