New o2 sensor went bad in 1 day after being

The connections to the sensor is heater voltage which is +12 volts supplied by external (from the ECM) wiring from the battery, and the 0 to + 5 volt self-generated signal from the O2 sensor is an input to the ECM. Suggest that you find another mechanic that isn't intent on selling parts. Also suspect is the quality of the replacement O2 sensor. It would take a major disaster within the ECM to make an input connection turn into an output and would likely damage the O2 sensor almost immediately--not a day later. Hope this helps!

I don't think your ECM is bad, just because it's putting out some volts or amps, that normal, how else would the computer be able to hold data in it, it be kinda like unplugging your clock radio at home, no voltage now you have to reset it. Just because they got a code for an o2 sensor, don't mean it's bad, a good tech. will have to check other thing before replacing a sensor, the code tells you where to start looking, the o2 sensor is like a 1 volt battery, with heat it increases voltage up to one volt and the computer will adjust fuel from that, the only way to bench test an o2, is with a torch and a voltmeter, heat up the senor and take a volt reading, might want to ask how they tested the sensor.Possible causes of this code, vacuum leak or catalytic converter.Bad ECM no.

You need to trust your mechanic or get a second opinion at another shop. Thats the only way to know forsure your getting the best advice. If we cant test, we cant a give advise. And yes its possible the PCM has a short internal and will need to be replaced.