Your reading should be obviuosly cyclical. If you are just wandering around in that area it isn't working. Did you get 70% key on engine off? Do you get 50% with O2 sensor disconnected , engine at idle? Does the duty cycle slowly go toward 0% if one puts the O2 sensor input to ground? Does the duty cycle go toward 100% if one grabs 12v with one hand and the O2 sensor input with the other?
Do this intergrater range test with O2 sensor disconnected and the battery through body and ground circuits hooked to the wire that goes to controller (not the sensor - hook where sensor hooked).
Bosch Master, ASE Master, L1
33 years MB technician