If you unplug the EHA while running, with no change, it is a pretty good indication that the EHA is not receiving a signal (mA current) from the ECU. Check for a duty cycle at the diagnostic port.

My meter only does Vac, but i did manage to measure across ports 2 and 3 on the diagnostic port and these are my findings: Key on, cold 7.5Vac. It's 10 degrees today so getting into closed loop is proving challenging without driving. I did notice with the engine cold and I unplugged the EHA it would stumble a bit and idle down (today). Then upon plugging back in, the idle would go up to about 1200, then would normalize, and finally really low idle <500rpms sounding like it wanted to die. If I gave it throttle input it seemed responsive and fine but would return to that low rubble idle. I'm sort of stuck on the idea of how it was driving perfectly then just decided to do anymore. Thanks for your thoughts so far Frank! I can try to wrangle up a meter that will show me duty cycle if that is needed. I've got a spare EHA from my 126 that seemed to work fine, can swap that over to test.

Just as an update, still tinkering around, but possibly have an idea what might be up. It seems to run perfect for about 10 seconds when you first turn the car on, then it'll stumble and try to die, and if you give it throttle it backfires in the intake. After a bit, I noticed my tach stopped working which made me think, well maybe my rear CPS sensor is going bad on me. Have that on order, will report back my results! I did unplug the sensor that lives under the fuel distributor intake boot, and it caused my idle to surge to 1000 and back down, so that leads me to believe that it's working fine. So next might be my potentiometer, really hope its not that!

Updates soon!