Hey z, I think ive figured it out thanks to your pics.
Please refer to to the diagram "egr.png" below.
As we all know, EGR opens at cruise to allow some exhaust into the inlet manifold, thereby cooling the combustion chamber so as not to produce as much NOx pollution.
Let'a describe the components in the diagram.
"A" is "the widget" and I believe theyre calling it an EGR Diagnostic Vacuum SWitch. I have it mislabeled here as EVRV.
a) one port with vacuum from line AB (red)
b) 2 black wires A1 "going to itself" (actually they connect the vac transucer on one side of the mechanism to the circuit board on the other side)
c) edge connector A2 which connects to the loom and therefore the ECU with 4 wire colours as shown.
d) 2 additional black wires AB that command the solenoid B
"B" is the solenoid. When it gets 12V on black wires AB it will open a port to let vacuum through from throttle body D to EGR C.
"C" is the EGR valve. When it gets vacuum on line BC it will open allowing exhaust from the manifold into the inlet manifold (not shown).
"D" is the throttle body which has a port that can be used to connect its vacuum D to B.
I *think*, based on research, the vacuum on line DB gets through to the EVRV which tells the ECU that the engine is asking for air/fuel. The ECU, depending on other parameters, optionally replies, commanding EGR.
The command causes 12V to get to the solenoid which activates, routing vacuum to the EGR, causing it to open. It stays open for as long as its needed.
Now where Im not entirely clear is how (or whether) the EVRV is giving feedback to the ECU. In one scenario it may expect vacuum at its port to change after its commanded EGR solenoid activation.
In any case, the ECU knows how to set EGR codes and logically the only way it can "know" is by examining the signals its getting on the white and/or grey wires coming from A2 (pink and black are simply hot-in-run and ground, respectively).
Based on my research, there are various EGR configurations out there of which this is just one. Some EGRs appear to have everything in one module while this configuration splits the valve, solenoid and sensory components out into three separate units. Cheaper if you need to replace one of them I guess.
So I think the next step is to pop it all on the car and see. Handily the 2 black wires AB were cut staggered so I can see which is which.
I suspect the reasons you're not seeing codes is that there is enough vac/wire connection to fool the ECU into thinking EGR is being commanded and complied with. This is essentially what Im going for.
I hope this helps someone!
Kudos as ever to zs.