O2 sensors

[sfkd90]

Hi all, I am having problem to detect the voltage change from "lean" to "reach" on my both upstream O2 sensors. Here are the tests I performed:

1) I back probed signal and ground wires while sensor is in the car and connected to the harness, but no reading on the millimeter.
2) When I removed O2 sensors and bench-tested them by applying torch while measuring the voltage, O2 worked as expected with signal fluctuating between ~ 0.1V to ~0.9V.
3) Finally, when I used a good scanner to check live data, no reading of upstream O2. The same scanner shows O2 sensors reading for my Jeep with no problems.
4) Finally, I replaced the old sensors with the new one and no results.

Does anyone have any clue why I see no response from front O2 even when I back-probe them?

Thanks!

[SuperHerman]

Not sure if applicable to your vehicle, but did you check any relevant fuses?