Multiecuscan

I am led to believe the O2 sensors on the Fiat 500 abarth which are 4 wire type use the Nernst principal fir measuring O2 concentration from 0 1 to 0 9v....I read 4 v and upto 5V.... This is intermittent.... Starts off in open loop with reasonable voltages.... Then goes high..... But it has on an occasion run perfectly in closed loop with the exoected 0 1 and 0.9v....Original fiat sensors used for replacement already...any clues to causes??