The smell that isn't there
Right now you probably don't notice anything at all. The furnace kicked on a few weeks back when the first cold snap came through the Ozarks, and it's been running steady since. No smell, no sound, nothing on the walls to tell you something's wrong. That's the whole problem with carbon monoxide. It doesn't announce itself. A gas leak has that rotten egg smell added on purpose. Carbon monoxide has nothing. No color, no odor, no warning sign except a headache that feels like you're coming down with something, or a sluggishness in the house that everyone just blames on winter.
Why Nixa homes are particularly worth checking
A lot of housing stock around Nixa and the greater Christian County area dates back to the building boom of the 1990s and 2000s, with a good number of older farmhouses and ranch homes mixed in from decades before that. Furnaces in those older homes have often been repaired piecemeal over the years rather than replaced outright. A cracked heat exchanger doesn't look like much from the outside. The furnace still runs. It still heats the house. But that crack is exactly where combustion gases can leak into the air your family breathes instead of venting outside like they're supposed to.
Winters here don't stay bitterly cold for months the way they do further north, but Nixa still gets stretches of nights in the teens and twenties, and that means the furnace runs hard during those cold snaps and then sits idle during the warmer breaks in between. That start-stop cycling puts stress on heat exchangers and seals. It's a pattern that's harder on equipment than a steady, consistent cold would be.
More on this from Why A Detector Is The Only Way Anyone Finds Out.
What a detector actually does that you can't
You cannot smell carbon monoxide. You cannot see it. You cannot feel it building up in a room. The only way anyone finds out there's a problem is a detector doing the one job it has, which is measuring something invisible and sounding an alarm before it reaches a dangerous level. This isn't optional equipment for a house with a gas furnace. It's the only early warning system that exists.
Check that you actually have one, and check where it is. A detector shoved in a closet or buried behind furniture in the basement isn't doing much good. It should be near sleeping areas, at roughly knee to chest height on a wall, not right next to the furnace itself where normal combustion byproducts might trigger false alarms.
What you can check yourself
Test the detector's battery the same way you'd test a smoke detector, by pressing the test button and listening for the alarm. Do this when the clocks change in fall, since that's also roughly when the furnace starts its real workload for the season. If the detector is more than five to seven years old, replace the whole unit rather than just the battery. The sensors inside wear out even if the device still powers on.
Look at the furnace exhaust vent or flue pipe from the outside of the house. It should be clear of leaves, nests, or anything blocking it, which happens more than people expect once the oak trees around Nixa start dropping leaves in October and November. A blocked vent pushes combustion gases back into the house instead of outside.
Where this stops being a homeowner job
If a detector ever alarms, get everyone outside first and call for help after, not the other way around. Don't open windows and try to air it out before you leave. Don't assume it's a false alarm because nothing smells wrong, since nothing will.
Beyond the detector check and the vent inspection, the rest is work for someone who does furnace repair for a living. A heat exchanger needs to be inspected with the right tools to find hairline cracks that aren't visible otherwise. Combustion efficiency and gas pressure need to be measured, not guessed at. If your furnace is original to an older Nixa home, or if it's had patchwork repairs over the years rather than a full replacement, it's worth having someone qualified look at the heat exchanger specifically, even if everything seems to be running fine. Fine is exactly how it looks right up until it isn't.