Help...o2 sensor p0420 2000 toyota camry

[theebigguy]

Please help...I can't remember the first code I got but my gas mileage tanked from a nice 29 highway to 20..Some messages advised to change your thermostat and get a new ECT.. I did that cleared codes and after a couple hundred miles gas mileage plummeted again... Now I get p0420... So I crawled underneath to hit the catalytic to see if we hear the rattle nothing. I purchased the car about a month ago from my mechanic and he said he put on a working used upstream denso o2 sensor. I did my homework and I get 12 volts to the heater and I get a reading of 4 ohms on the heater wires. So the heater part is working. I put a ohm meter on the o2 side and I get a steady 3 volts..No fluctuation on the meter ..If I rev up the engine nothing changes 3 volts.... Now if I disconnect the o2 sensor and check the voltage on the car side of the o2 plug I read 3 volts.. Here's my question. Should the car be sending 3 volts to the o2 sensor? This is a California 2000 toyota camry le 4 cylinder... I'm just trying to make sure it's the o2 before I throw out a $100.00...Thanks in advance

Thebigguy