With only the green wire connected to the ECU I get a pretty stable .52 volts, which I have read in the archives to be the ECU reference voltage. Hopefully all is well at that end. At no time does voltage sweep from .3 to .7 v. That is the right resistance on the heater. Before you replace sensor warm up engine attach voltmeter to O2 sensor lead and then momentarily pinch rubber return line behind fuel pressure regulator.
V then look for something else making it lean.
A faulty sensor can give you the low V reading but an air leak or defective MAF sensor can do the same. The heated sensors are used to keep the signal active while idling. Idling in cold weather primarily, the sensor will fall below its 600F temp at which the reaction that makes it output voltage begins to work. If it were to stay above this temp the non-heated would be okay. There are different sensors out there also.