Our furnace went on sabbatical today, and I learned something from our HVAC friend that I didn't know (and didn't find in my 4 minute online search before I called said friend).
On newer furnaces there is usually a blinking status light on the control board. If something goes wrong, see if that light is blinking in a pattern. Watch for a few repeats so you know the pattern for sure, then check - there is usually a sticker/label inside the furnace with a key for the status lights.
Somehow I feel like I "should" already have known this (I'm semi-handy and have repaired our washing machine on my own...) but I didn't & I didn't see it in my search of this forum. Hope it can save someone some $
(insert face-punch for not having gone furnace-free... but it is below freezing here today, so maybe wait until a warmer day for the punch...)
Our specific issue was a pressure switch, it is very windy today and our exhaust doesn't have an elbow on it (it will tomorrow), so if the wind is blowing into the exhaust our gas furnace won't fire up because it won't be able to vent properly. HVAC friend reset the switch by turning off the furnace for a few minutes, then back on - pressure switch read okay, and furnace fired up.