'04 SR5 V6 exhaust sensor location

[jongor]

I'm getting an error code, "P0051 - HO2s bank 2 sensor 1, heater circuit low".

I saw in another thread here that it's possibly an air/fuel sensor and not an O2 sensor.

I need to know which sensor it is, a little research says there's 4 (2 before the cat and 2 after). And if it is in fact an air/fuel sensor, is that a different part number from the O2 sensor.