That first cold morning tells you a lot
If you live around Republic, you know how fall works here. It stays mild into October, sometimes warm enough for the AC to still run, and then a cold front drops out of the Plains and overnight you need heat. That first real cold snap, usually somewhere in that stretch between late October and mid-November, is when a lot of furnaces show their hand. If yours has been sitting unused since March, this is the moment any problem that built up quietly over the summer decides to become your problem.
The complaint is almost always the same. The thermostat is set to 68, the furnace kicks on, and either nothing happens, or it runs for a few minutes and shuts off, or it runs but the air coming out is barely warm. None of that is random bad luck. It is usually the direct result of eight months of the system sitting idle in a Missouri basement or crawlspace, collecting dust, humidity, and the occasional mouse.
Why the first cold snap is the hard test
Southwest Missouri swings hard between seasons. Summers here are humid, and that humidity does not stay outside. It gets into ductwork, into the furnace cabinet, onto electrical connections. Add a summer's worth of dust from windows open on the nice days, plus whatever gets kicked up by lawn mowing and harvest dust drifting in from the surrounding farmland, and you have a filter and a blower wheel that have been quietly loading up for months.
Then the first cold night asks that dirty, humidity-stressed system to fire on the first try and run for hours straight, something it has not done since last winter. Igniters that were already weak give out. Flame sensors coated in a fine layer of carbon fail to detect a flame and shut the system down as a safety measure. Capacitors that were marginal all summer finally quit under the load. It is not that the furnace broke overnight. It is that it was never really tested until now.
More on this from Why Furnaces Fail In The First Cold Snap.
What you can check before you call anyone
Start with the filter. In most Republic homes, especially older ranch houses and split-levels from the 70s through 90s that make up a lot of the housing stock here, the filter sits in a slot near the return duct or inside the furnace cabinet itself. If you cannot remember the last time you changed it, that is your answer. A filter clogged with a summer's dust will choke airflow enough to trip the furnace's safety limit switch, and it will do it repeatedly.
Next, check the thermostat batteries if it is battery powered, and make sure the switch on the furnace itself, often mounted on or near the unit and easy to mistake for a light switch, is actually in the on position. It is more common than people think for that switch to get bumped during summer storage of holiday decorations or lawn equipment nearby.
Look at the vents and registers throughout the house. If furniture got rearranged over summer, a couch or bookshelf might now be blocking a return air vent, which starves the system of airflow the same way a dirty filter does. Also glance outside at the intake and exhaust pipes if you have a newer high-efficiency furnace. Wasps and mud daubers love building nests in those pipes over summer, and a blocked exhaust will shut the unit down for safety.
Where this stops being a DIY job
If you have changed the filter, confirmed the switch is on, cleared the vents, and the furnace still will not stay running, or if it starts making a rhythmic clicking without ever lighting, or if you smell anything like burning dust that does not clear after a few minutes of runtime, stop there. Those are signs pointing at the ignition system, the flame sensor, or the electrical components inside the cabinet, and none of that is something to troubleshoot with the panels off.
Gas furnaces add a real safety line you should not cross. If you ever smell gas, hear a hissing sound near the unit, or see any scorch marks around the burner area, shut the system off at the switch, and if you smell gas strongly, leave the house and call your gas provider before anyone else. That is not a maintenance issue, it is a safety issue.
Getting ahead of next year's cold snap
A lot of the failures that show up in that first cold week were preventable with a look in September, before the cold arrives and before every repair technician in Republic is booked solid with the same first-cold-snap calls everyone else is making. If your furnace is more than twelve or fifteen years old, which describes a good number of the systems still running in homes built before the town's recent growth, it is worth having someone check the heat exchanger, igniter, and flame sensor at the end of summer rather than waiting for the system to tell you itself, at 6 a.m., in the dark, in December.