I thought it was the O2 also, when I unplug the O2 the voltage goes back to 4.0V when the car is warmed up. Checked the wireing also and everything is fine.
When I ground the O2 connector the voltage goes to 0V so everything works the way it should.
Also once I unplug the O2 and plug it back in the voltage drops to 4 and slowly climbs up to 7.22V, I can't get it to stop anywhere before that.
I'll try a different O2, this one is relatively new but it could be bad.
Anything else I could try?
1985 190E 2.3L - a constant project.