328 Slow Down Light Self Test

My slow down light doesn't light up for the self test upon starting, and I am wondering whether it is the slow down ECU or the "protection relay for control system" that controls the self test. I have cleaned all the connectors and grounds, and verified (i) that the slow down light comes on if I ground the red wire from the slow down ECU connection block, (ii) that the blue/black wire has 12V when the key is on, (iii) that the double black wires go to ground, and (iv) that the thermocouple is producing zero mv at room temperature. Are there any other tests I can run? I would like to know which device has the self test circuit, which I assume is a very brief timing circuit to lift the self-test ground after power is applied for a couple of seconds. With the relay being expensive and the ECU being ridiculously expensive, I hope there is a way to pin this down without buying a part I may not need. TIA.

--Charlie