A while ago (like a year or more) there were some threads floating around about O2 sensor voltage readings in the 3+ volt range on diagnostic scanners. I don't recall there being a clear answer as to the reason at the time. I have had some O2 sensor issues recently so I went digging for info and basically found that the reason for the trouble was that I bought the wrong type of sensor. I bought an O2 sensor, when what I needed was an Air/Fuel Ratio sensor. The problem was on a Toyota, but the same principles of how the sensors differ apply to every brand of vehicle. The link below comes from a website we use in school, and has tons more info about various sensors and their functions, as well as dozens of other tech articles that can be useful during diagnosis of a problem.
What my issue boiled down to is the ECU was getting a reading of incredibly lean mixture from the O2 sensor. Because on an O2 sensor rich = high voltage and lean = low voltage. but wih an A/FR sensor its the opposite, rich = low and lean =high. The ECU would add more fuel and get a rich voltage back rom the sensor,except the ECU is programmed to read high voltage as lean, even though the sensor was responding as it should. And so the cycle repeats itself until the ECU can no longer adjust, which in this case pegged the Long Term Fuel Trim at 44.5%.
But back to the point, if you get oxygen sensor voltage reading in the 3V range on a scanner, its because you have an Air/Fuel Ratio sensor, which gets supplied a reference voltage of 3.3V from the ECU. The sensor then bumps up (if lean) or bumps down (if rich) the return voltage signal back to the ECU. Some ECUs convert scanner output to a normal O2 sensor output voltage to avoid confusion, but not all.
Be sure of what you have before purchasing a replacement sensor.
Edited by Fairtax4me, 15 February 2013 - 07:46 PM.