Few things are more frustrating on a hot Michigan day than an air conditioner that's clearly running but not doing its job. The compressor is humming outside, the vents are blowing air, the thermostat is set where it should be and yet the house is 80 degrees and stuffy.
The good news is that this is a diagnosable problem, and several of the most common causes can be identified with a quick self-check before you even pick up the phone. The bad news is that others require a technician and shouldn't be deferred, because running an AC system with certain issues can cause compressor damage that turns a manageable repair into a major one.
Here's a systematic walkthrough of what causes this situation and what each scenario means.
Start With the Simple Stuff
Check the Air Filter
This sounds too obvious, but a severely clogged air filter is responsible for a surprising number of 'AC not cooling' service calls in Michigan every summer. The air filter is the primary protection for your evaporator coil the indoor component that actually performs the cooling. When the filter is so clogged that airflow is severely restricted, two things happen: the system can't move enough air to cool the space effectively, and the evaporator coil can freeze over, which makes the problem dramatically worse.
Pull out your filter and look at it. A brand-new filter is usually white or light gray. A filter that needs changing is visibly gray or brown, and you can't see light through it clearly. If it looks like this, replace it immediately, turn the system off for 30 minutes to let any ice on the coil melt, and then restart. If this was the issue, cooling should improve within an hour.
Standard 1-inch filters should be replaced every 1–3 months depending on your household. Thicker media filters can go longer, but check the manufacturer's recommendation. During peak cooling season, checking monthly is a good habit.
Check the Thermostat
Confirm the thermostat is set to COOL (not just FAN or HEAT), that the temperature setting is below the current room temperature, and that the batteries aren't low. Smart thermostats can sometimes get stuck in a bad state after a power fluctuation a quick reset (usually pulling it off the wall and reseating it) can clear these issues.
Check the Outdoor Unit
Go outside and look at the condenser unit. The fan on top should be spinning. If the unit is running but the fan isn't spinning, you likely have a failed capacitor or fan motor this is a relatively common repair and one that should be addressed immediately, because a condenser without a working fan will overheat quickly.
Also check whether the unit is blocked. Condenser units need clear airflow on all sides. Overgrown shrubs, fences placed too close, or debris accumulated around the unit (common after Michigan windstorms) can restrict airflow and significantly reduce efficiency. A few feet of clearance on all sides is recommended.
The Frozen Evaporator Coil
If you checked the filter and it looked fine, but you notice ice forming on the refrigerant lines running into or out of your air handler, or if the unit seems to be running but producing very little airflow, a frozen evaporator coil is a likely culprit.
This happens when airflow over the coil is insufficient (from a clogged filter, blocked vents, or a failing blower motor) or when refrigerant levels are low. In either case, the coil surface gets too cold and moisture in the air freezes on it, eventually forming a thick layer of ice that blocks airflow entirely.
The immediate fix is to turn the system to FAN ONLY or shut it off completely and let the ice melt this can take 1 to 3 hours. Once the coil has thawed, the system can be restarted. But the freeze was a symptom of something else, and without addressing the underlying cause, it will happen again. If the filter was fine and the vents are all open, the issue is likely refrigerant which requires a technician.
Low Refrigerant
Refrigerant is the substance that makes air conditioning physically possible. It cycles between the indoor evaporator coil and the outdoor condenser, absorbing heat from your home's air and releasing it outside. When refrigerant levels are low, the system loses the ability to perform this heat transfer efficiently.
Refrigerant does not 'get used up' under normal operation. If your levels are low, it means there's a leak somewhere in the system. Adding refrigerant without finding and repairing the leak is a temporary fix levels will drop again, and you'll be back to the same problem.
Low refrigerant is something only a certified HVAC technician can diagnose and address. Refrigerant handling requires EPA Section 608 certification, and for good reason refrigerants are regulated substances. The technician will check pressure levels, identify the source of the leak, repair it, and recharge the system to the correct level. This is not an optional repair: running a system low on refrigerant puts serious stress on the compressor, which is the most expensive component to replace.
Dirty Evaporator or Condenser Coils
Both the evaporator coil (inside) and the condenser coil (outside) are heat exchangers they work by allowing refrigerant to transfer heat through their surfaces. When those surfaces are coated with dirt, dust, or debris, heat transfer becomes less efficient and the system has to work harder to produce the same cooling effect.
Condenser coils, located in the outdoor unit, are particularly exposed. Michigan's cottonwood season in late spring sends large amounts of organic material into condenser fins, and summer storms bring leaves and other debris. A coil that's partially blocked by debris can look relatively clean from the outside while having significant buildup on the interior fin surfaces.
Evaporator coil cleaning is typically part of a full AC tune-up. Condenser coil cleaning can sometimes be done with a gentle rinse from a garden hose (from the inside out, not outside in), but significant buildup usually benefits from professional cleaning with appropriate coil cleaner.
A Failing or Undersized System
Sometimes 'not cooling properly' isn't a malfunction it's a capacity problem. If your system runs continuously on hot days without reaching the thermostat setpoint, but the system checks out mechanically, it may simply be undersized for your home's cooling load.
This can happen because the original system was incorrectly sized, because your home has added square footage or changed significantly (new windows, added insulation, a finished basement or addition), or because the system has lost efficiency to the point where its effective capacity is significantly below its nameplate rating.
The right approach here is a professional load calculation an honest accounting of what your home actually requires versus what your current system can deliver. This assessment informs whether the system needs repair, replacement, or supplementation.
When to Call for AC Repair and What to Tell the Technician
If you've worked through the self-check items above and the system is still not cooling properly, it's time for a professional diagnosis. When you call, the more detail you can provide, the faster the technician can prepare. Useful things to mention:
• When the problem started and whether anything changed around that time (storms, power outages, filter replacement)
• Whether the outdoor unit is running and whether the fan on top is spinning
• Whether you've noticed ice forming anywhere on the system
• What your typical bill has looked like this season compared to previous summers
• The approximate age of the system if you know it
A good technician will use this information to direct their diagnosis efficiently. The most common repair scenarios capacitor failure, refrigerant leak and recharge, contactor replacement, coil cleaning are typically resolved in a single visit with parts that experienced HVAC companies carry on their trucks.
What you want to avoid is deferring a diagnosis once you've identified that something is wrong. A frozen coil that's ignored becomes a damaged compressor. Low refrigerant that goes unaddressed leads to the same result. The window between 'something seems off' and 'significant repair bill' is often shorter than people expect during peak cooling season.