Ok, I now have the equipment failure LED functional on my Vision PRO IAQ with EIM system. That was connecting the York X/L to the EIM's L input. Doing so revealed the existence of the Thermostat's Red LED behind the white plastic case, top right.
Now I read that the Emergency Heat is supposed to illuminate this or a similar LED. But it doesn't. I do get "Aux Heat On" displayed on the LCD instead of simply "Heat On".
Does the Vision PRO IAQ thermostat inherently operate the LED under emergency heat or is there some other wiring necessary?