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Today I had a tuning session for a S13 with modifications. I have made chip for the same car few years ago. Latest changes for the car was RB MAF to Z32 MAF and that was the reason for new tune and there was also now NIStune equipped ECU available for the car to use. Everything was fine when the O2 feedback was disabled from feedback switch. AFR was roughly around 15 on idle. At this point I enabled O2 control and after a few seconds the car stars to go towards 20 and above. I notice O2 voltage being over 1 volt. In the very beginning after turning O2 feedback on, I noticed values above 1,5 also. So it was not possible to put feedback on as the AFR was corrected completely to incorrect direction.
Later we went to test with another ECU which has been in my own car and it has been working perfectly fine. With this the same issue again.
We did measured signal from PLX wideband 0-1volt output to be 1 volt and AFR was 14 at that point. Did measure also from ECU pin and the voltage was the same. Enabled the O2 feedback from NIStune and on PC the voltage was something like 1,6 volts and voltage meter still on 1. AFR started to go leaner and the reading on voltage meter going towards 0, but on PC the lowest was maybe 0,8 volts.

What could cause this? I was told that the car was running the previous day without problems with the RB MAF and everything else the same. Car was also taken to MOT with the same setup with the RB MAF.

What can cause O2 voltage being roughly 0,6 volts higher in ECU value than it actually is in the wire?

Another thing... this was tested also with my WBo2 with the same result. That wideband unit was used few weeks earlier on RB25DET to have narrowband signal.

I really cannot think of anything but having some grounding issue, which also does not make much sense if everything is OK with the old ecu without NIStune card in it and tune for the setup with RB maf.

Not sure with the voltage reporting, but Nistune reads Nissan consult O2 input and that reports what it sees on the line against sensor ground. Check the grounding you are measuring against

Regardless that doesn't make much difference since the voltage should be swinging around 0.5 volts (say 0.2 min, 0.8 max) so 1V or 1.5V either way is too high so you may need to calibrate the 0-1V (Which is what it should be) output on your wideband to suit this range until you can see the narrowband fluctuating between 0.2 - 0.8 in a steady wave form when at 14.7:1

This ended up being a grounding issue. Ground connection to intake from battery was cleaned, after that, the signal inside ecu was correct. Wideband unit was located behind radio and grounding made from there, so there was some ground potential difference between the wideband unit and ECU. Signal would probably have been more than 1 V if it would have been measured against engine ground instead.