What you are probably noticing first
Most people do not go looking for furnace trouble. It finds them on the first genuinely cold night of the year, when the thermostat is set where it has always been set and the house never quite gets there. A furnace rarely fails all at once. It spends a season telling you it is going to, and the tell is usually small enough to explain away.
The common ones around Nixa are a burner that fires, runs for a few minutes and shuts off again before the house warms up, one room that stays noticeably colder than the rest no matter how the vents are set, and a system that runs far more often than it did last winter for the same result.
Sounds a furnace should not be making
A furnace has a normal noise. It is a click, a whoosh, then steady air. Learn that sequence in October and anything new in January stands out.
A delayed bang or boom at startup means gas is pooling before it lights, and that is worth attention rather than patience. A metallic scrape usually means a blower wheel is touching something it should not. A rattle that comes and goes with the fan is often just a loose panel, which is the one on this list you can reasonably chase yourself. A high squeal when the blower starts points at the motor rather than the burner.
More on this from furnace repair in Nixa.
What an Ozarks winter actually does to a furnace
This region is hard on heating equipment in a specific way. Winter here is not steadily cold, it swings. A week in the fifties followed by a night in the teens means a furnace that has been idle then asked to run hard, over and over, and that cycling is what finds a weak igniter or a tired blower motor.
The humidity does its share too. A furnace sitting unused through a Missouri summer sits in damp air, and moisture is unkind to a heat exchanger and to anything electrical near the burner assembly. A lot of the trouble that shows up in December was set up in July.
Outside the city limits, plenty of homes around Christian County run on propane rather than natural gas, which adds its own failure mode: a system that starts fine and quits in the coldest weather is sometimes a regulator or a supply problem rather than a furnace problem at all. Worth knowing before you pay somebody to look at the wrong thing.
Also worth reading: see the full details.
The checks worth doing before you call anyone
Start with the filter, because it is the cause often enough to be worth ruling out first and it costs almost nothing. A clogged filter starves the system for air, which makes it run hot, shut itself off early and cycle exactly the way a failing furnace does. If yours is grey and you cannot remember replacing it, replace it and give the system a day.
Then check that supply vents are open and not behind furniture, and that return air grilles are clear. Restricting the return is the quiet version of the same problem. Check the thermostat batteries if it takes them, and check that nobody has nudged it to fan-on, which makes a house feel drafty and cold while the furnace is behaving perfectly.
If you can see the burner flame safely without removing anything, it should be mostly blue and steady. Yellow, lazy or flickering flame is not a homeowner adjustment. It is a reason to stop and call.
There is more on that at furnace repair services in Nixa.
The line you should not cross
Anything involving the gas valve, the heat exchanger or the flue is not a homeowner job, and not because of liability language. A cracked heat exchanger can put combustion gas into the air you are breathing, and the symptoms of that are vague enough to be mistaken for a winter cold.
If your house does not have working carbon monoxide detectors on every sleeping level, that is the single most useful thing you can do this week, ahead of any furnace decision. They are inexpensive and they fail safe.
Repeated lockouts are the other clear stopping point. A furnace that shuts itself down and needs resetting is not being temperamental, it is refusing to run because something told it not to. Resetting it repeatedly overrides a safety decision without fixing what triggered it.
Repair or replace, honestly
The question everyone actually wants answered is whether to spend on a repair or start over. There is no clean rule, but there is a useful way to think about it: weigh the cost of the repair against how many more winters the rest of the system is likely to give you, and be honest about the second number.
A furnace in its first decade with one failed component is usually worth fixing. One well past that, on its second or third significant repair, running through Ozarks temperature swings every year, is a different conversation. Get the diagnosis in writing, get a second opinion if the number is large, and be suspicious of anyone who arrives at replacement before they have explained what failed.