The front O2 .
I'd bet that this is the one that set your light, so replacing it before purchasing the other sensor could save you money.
Totally inaccurate.
I've said dumber things on here.
I agree with knowing the codes to know what is actually going on. I've seen a lot of guys replace an O2 with a P0171 code. All they did was throw money away because that code is 99% never about the sensor.