EHA does only one thing. It is a controlled leak with a limited flow rate.
The way to evaluate EHA is to watch the differential pressure as the EHA current is altered. Total lambda correction of -12ma to +12ma can be viewed by first grounding the O2 sensor input and then grabbing the O2 sensor input with one hand and battery 12v with the other. the control unit sees 0.0v one time and about 1.5-2.5v the other. It causes the control to do the current changes and they will cause about a .2 bar total differential pressure difference one side to the other. (differential pressure should go from .3 bar to .5bar with .4bar being the 0.0ma position.
Bosch Master, ASE Master, L1
33 years MB technician