According to the guy at the Toyota parts desk, bank 2 is right behind the radiator, and my Haynes manual hints
that's bank 1
I can see the sensor sticking out of the manifold back by the firewall, but getting to it is a different story. Then there is one near the cat, which is called the "downstream" sensor.
The whole thing about removing the seat really put me off the whole thing, but when I was checking for the connection under there, I couldn't see anything but the connections for the power seat, seat heater, and the DC/AC power converter. It must be under the carpet.
Thank you very much for your reply, it was much faster than I expected!
Oh, BTW, the sensor cost $207.00.
That seems a bit excessive to me, but then, WTF do I know.