[duffyatkinson]

I think what happens is that the ECU finds fault on a comparative basis, i.e. the old sensor, while still within parameters, is registering values so far away from the new one that the ECU determines this to be a fault all on its own.

Also note that the O2 sensors on our Tundras are the 4-wire type, wherein 2 wires are used to bring the unit up to operating temperature and the other 2 are the data communications circuit. It's possible that a differential in operating temperature(s) vs. time can throw a code as well.

Don't be surprised to find that you will need to replace all four to keep CEL codes from popping up. And stick with quality Denso or NGK sensors. If you are doing your own R/Rs, hit the base on the front ones and the nuts on the rears with a good blast of WD-40 or PB Blaster beforehand and let it sit for an hour or so to help with removal. Make sure to replace the flange gasket on the rears.