You can not have closed loop without EHA current doing it, that is why I suggested to look at it.
The O2 sensor can be going cold at idle if the heater isn't working.
Try disconnecting the O2 sensor under the carpet passenger side and see if the car still needs the EHA disconnected. The point here is that when warm the EHA current should be zero with the O2 sensor disconnected. If it still matters whether the EHA is connected you do have a problem there.
Bosch Master, ASE Master, L1
33 years MB technician