Most furnaces are incredibly amazingly simple and the large PCB is just for reliability and to support the transformers.

Many even now are (blower on/off) and (flame on/off). Only the high end ones have variable blowers and flame heights.

I know this because before ours was replaced I had to jumper past a bunch of the board to keep it running.

>Many even now are (blower on/off) and (flame on/off). Only the high end ones have variable blowers and flame heights.

There's a few other safety checks the board will do.. check pressure switche to ensure there is draft before attempting to open the gas valve and light off. There's also thermal limit switches to make sure things don't get to hot if your main blower goes out.

I’d have to check my new one but often those switches are just in series as a physical disconnect.

On some boards they're inputs as that's what provides the necessary info for the board to provide flash/error codes. Not unusual to see additional standalone limit switches (like what you observed) for extra protection.