Lots of possibilities, but since you are trying to deal with fuel control, that sounds like a good place to start.
The #3 pin duty-cycle indicates the amount of time the frequency vale is open (leaking off lower chamber pressure). What is most important is that it is leaking. Screw the O2 sensor, until you get the freq valve working. Can you hear or feel the valve buzzing? IT HAS TO BUZZ. With the O2 sensor disconnected you should have a 50/50 duty cycle. You must get that working first.
I think the #3 pin is the same as the ground side of the valve. BTW the #1 cause here of no buzzing is a bad OVP (the first year of OVPs). I think the power for the valve comes from the fuel pump relay and should be hot whenever the fuel pump is working.
I would check it all at the valve. One side is hot the other side grounds out the 50/50 duty cycle (that means that the valve is open 50% of a cycle and closed 50% of the cycle).
Once it is buzzing at 50% adjust the mixture. Even though any idle mix is achievable with with the mixture adjustment the car won't get enough fuel under load unless the freq valve buzzes.
Bosch Master, ASE Master, L1
33 years MB technician