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Here is a pic from my RX350, which shows the O2 sensor at zero until engine warms up, I believe this is because it includes a heater that needs to get to temp before it registers. O2 sensor is after the cat, before the cat is the AF sensor and it starts at 3+ volts. So I would expect some variation after 5 mins or so. If heater circuit was bad you would get a CEL I think.
S12 = Bank 1, sensor 2 (O2)
S21 = Bank 2, sensor 1 (AF)
BTW, this is from an engine with zero compression on cylinder 4 (bank 2), but the O2 and AF sensors were fine. Engine would idle roughly until bank2 O2 sensor got pissed off and changed the fuel trims when going into closed loop, which caused it to stall.
