What you're probably noticing right now
The afternoons are still hot enough in Nixa to keep the AC running hard, but the mornings have that first hint of cool that tells you fall is coming to southwest Missouri. Maybe you've noticed the unit running longer than it used to, or cycling on and off more than you remember from July. Maybe there's a musty smell when it kicks on, or a room that just doesn't cool the way it used to. This is the window to deal with it. Once the Ozarks get their first real cold snap, usually sometime in October or November, you'll want your system to have already earned its rest, not be limping toward it.
Why this matters more here than in a lot of places
Nixa summers are long and humid, which means a lot of area homes run their AC hard from May through September. That's five months of near-constant cycling, condensation, and dust pulling through the system. Add in the fact that a good number of houses in this area, especially in the older parts of town and out toward Christian County, still have units that are ten, fifteen, even twenty years old. Those systems weren't built to run flawlessly forever, and a summer's worth of strain shows up right about now, at the point when you're asking the unit to make its last hard push before you shut it down for the season.
More on this from Redeemed HVAC.
What you can check yourself
Start with the filter. If you haven't changed it since early summer, it's probably overdue, and a clogged filter is the single easiest way to make a system work harder than it needs to. Look at the outdoor unit too. Grass clippings, cottonwood fluff, and leaves pile up around the coils in this part of the year, and that buildup blocks airflow. A gentle rinse with a garden hose, with the power off, can make a real difference.
Inside, check that your vents aren't blocked by furniture or rugs, and that the area around your indoor unit or air handler is clear. Look at the drain line near the indoor unit for any signs of a slow leak or standing water, which is common in humid climates like ours when condensation lines get a little algae buildup over the summer. If you have a programmable thermostat, this is also a good time to check the battery and make sure it's reading temperature accurately, since a bad sensor can make a healthy system seem like it's struggling.
Also worth reading: see the full details.
Where it stops being a homeowner job
If the system is short cycling, meaning it turns on and off every few minutes instead of running a full cycle, that's not something to chase down yourself. It usually points to either an electrical issue, a refrigerant problem, or a failing part inside the unit, and none of those are safe or practical to diagnose without the right gauges and training. The same goes for any burning smell, a breaker that trips repeatedly, or ice forming on the refrigerant lines even in warm weather. Ice on an AC line in September sounds backwards, but it's a common sign of restricted airflow or low refrigerant, and it needs a trained eye.
If your energy bills have crept up over the summer without a clear reason, that's also worth having someone look at, since it often means the system is working harder than it should to do the same job. A tune-up before cold weather sets in gives a technician the chance to catch a weak capacitor, a dirty coil, or worn contactor before it turns into a breakdown you notice on the first genuinely cold morning, when you're switching the whole house over to heat and asking parts of the system to do double duty.
There is more on that at AC tune-up and repair.
Getting ready for the switch to heat
A lot of homes in the Nixa area run heat pumps rather than separate furnaces, which means the same outdoor unit that cooled your house all summer is about to start heating it. That makes an end of season checkup even more useful, since problems in the outdoor unit affect both sides of the year. If you have a furnace instead, this is still a good time to think about it, since a technician looking at your cooling system can often flag furnace issues while they're already in the mechanical room or attic.
Whatever your setup, the goal is simple. You want the system checked and cleared before the first cold snap forces the question. It's a lot easier to deal with a sluggish AC on a mild September afternoon than a dead heat pump on a 28 degree morning in November, when everyone else in Christian County is calling for the same reason.