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Bank 1 Sensor 1 is not the rear. It's the front. Bank one is the closes to cylinder 1 which in your case is the passanger side, sensor one indicates it's the first sensor on that side. So passanger side 1st sensor (before the catalytic converter).
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It's actually very simple to replace. it's screwed into the exhaust manifold, you will need an oxygen sensor socket. Spray down the old sensor using WD-40, unhook the electrical connection and unscrew it using the socket , and extention and a rachet.
I hate to over post someone else but I see that you are on line and the last expert isn't. Like he said, B1S1 is not a rear sensor (behind cat) but a front sensor (in front of cat). It is, however, in the rear most bank (nearest to the firewall - not the passenger side. It is NOT easy to access. You have to get at it from underneath the van unless you remove the entire cowl assembly that the wipers are located in. It is extremely tight under there even with a lift an some sockets won't fit. I have even had a few sensors get stuck in the manifold on the way out and had to remove the entire exhaust manifold the complete the repair. It would caution you against trying this yourself unless you are very mechanically inclined. I also wouldn't worry about the sensor behind the cat just yet. There are 3 sensors and usually once one goes, they all go, but why spend the money yet? If any of the other sensors go bad, the check engine light will come back on and you can repair as necessary. Autozone will pull the codes for free if you don't have a scanner. One last piece of advice; make sure to spend the extra money on genuine Toyota sensors; I can't tell you the number of cars that come to the shop with after market sensors that don't work. If you would like you can question a moderator here at justanswer.com and see if OnlineMechanic and I can split your payment when you accept because, again, I don't want to interrupt someone else's answer.