[ImperialCrown]

Has it ever set O2 sensor fault codes? The PCM is a better tester of the O2 sensors than any manual test, which is more of a general test.
The downstream may straighten out after a long drive. See what its range is after it's hot and working for awhile. The downsteam O2 will change voltage up/down at about half the rate of speed than the upstream O2 sensor.
The O2's are inactive while cold, so I can't say whether 0 volts is bad or not. The PCM doesn't use the O2 sensor and the O2 heater should be on full until closed loop (~600°F/316°C) anyways, so I don't see the element cold test being a validation.

 

[NssJ]

I just re-read it, its meant for scan tool voltage testing, not backprobing, so I couldn't test it right anyways.
I did also check the heater circuits. both were at 5.5ohms (5-7 good), so mostly alright.

There's no codes for them, they just share some voltages with other sensors that may be knocking out the bus, so anything that's not playing a long will have to go. These the first parts to stand out as iffy, so wanted to ask