You must resolve the eHA current issue. 20ma is appropriate for key on, ENGINE off readings. The next step is to disconnect the O2 sensor. The ma reading should be zero with the engine running!!. If it is different this issue must be resolved.
If you have 0.0ma then try grounding the computer side of the O2 connection. This should cause the ma to slowly correct to the maximum which is about 12ma. Next place one hand on twelve volts and hold the computer side of the O2 connection with the other hand. (this applies a small >1v signal to the controller) You should slowly go to -12ma, full correction lean.
If the system has the ability to do this, the next step isn't so easy. The next step for testing would be to watch the differential pressure and to verify that it is .4bar and that it moves .1bar greater at 12ma and goes .1 bar less at -12ma.
Control is then working. See what you get with the O2 disconnected.
Bosch Master, ASE Master, L1
33 years MB technician