Hi Allan,

Your reading is normal for the sensor fitted to that vehicle, they are Toyota's early version of AFR sensors and not narrow band sensors as you where expecting.

It sounds like the cat is foo barred. The ECU will probably switch off the OBD monitoring with the heater faults being present, but now you have fixed them it has started doing its job again and the P0420 code is the result.

Check all the normal bits and bobs as far as cat efficiency faults go but you will probably need a new cat .

Regards Mark