Having the same issue - the old (dumb) thermostat worked fine for years, then stopped. You hear the heat turn on, but the blower motor never engages. I installed the Nest 3rd gen, and it recognizes the three components - power, heat, fan. The weird thing is that if you test each separately, (sometimes) they work. but never together. Even stranger, is once it thinks it's heating, it shows the fan as active when it is not.
I've gone through the entire wiring, board, connection options, and I cannot find a cause for it.
