There’s a specific kind of frustration that comes with an AC that runs all day but can’t get your home below 82°F. You can hear it running. You can feel the air coming through the vents. But the temperature just won’t budge.
This is one of the most common AC complaints we hear from McAllen homeowners, especially in the middle of summer when the system is being pushed to its limits. The good news is that there are a handful of root causes that account for the majority of these situations, and once you know what you’re dealing with, you can make an informed decision about what to do next.
Start with the Easy Checks
Before calling a technician, a few quick checks can rule out simple causes:
Check your air filter. A clogged filter restricts airflow through the system, which reduces cooling capacity and can cause the evaporator coil to freeze up. If your filter is dirty, and in South Texas, filters can get clogged faster than you’d expect with all the dust, replace it and see if that makes a difference within an hour or two.
Check your thermostat settings. Make sure it’s set to COOL, not FAN ONLY. Also double-check that the temperature is set lower than the current room temperature. It sounds basic, but this catches a surprising number of “AC not cooling” calls.
Check all your vents. Make sure every vent in the house is open and nothing is blocking airflow. Closing off rooms or covering vents might seem like a good way to redirect cooling, but it actually throws off the pressure balance in your duct system and reduces efficiency.
Check the outdoor unit. Step outside and take a look. Is the outdoor unit running? Is there ice forming on the refrigerant lines? Is there obvious debris — leaves, grass clippings, debris from landscaping, packed against the unit blocking airflow? A dirty condenser coil or blocked unit can significantly reduce cooling capacity.
Common Causes When the Easy Checks Don’t Help
Low Refrigerant (Most Common)
If your filter is clean, settings are correct, and the outdoor unit is running, but the air coming from your vents is room temperature instead of cold, refrigerant is the most likely culprit.
Air conditioners don’t consume refrigerant the way a car burns gasoline. If the level is low, there’s almost certainly a leak somewhere in the system. Common leak points include the evaporator coil, the refrigerant lines, and the connections at the unit.
This is not a DIY fix. Handling refrigerant requires EPA certification, and recharging without finding and fixing the leak will just result in the same problem in a few months. A technician will need to locate and repair the leak and then recharge the system to the correct level.
Dirty Evaporator or Condenser Coils
Your AC system has two coils: the evaporator coil (inside, in the air handler) and the condenser coil (outside, in the big unit). Both need to be reasonably clean to transfer heat efficiently.
In South Texas, condenser coils accumulate dust, pollen, and debris year-round. When the coil surface is coated, the system can’t shed heat effectively, and cooling capacity drops. A coil cleaning, which a technician can perform during a tune-up, often makes a noticeable difference.
Frozen Evaporator Coil
If you notice ice forming on the refrigerant lines near your air handler, or the airflow from your vents suddenly drops to almost nothing, you may have a frozen evaporator coil. This happens when airflow is severely restricted (clogged filter being the most common cause) or when refrigerant levels are low.
If you suspect a frozen coil, turn the system to FAN ONLY (not cooling) to let the coil thaw for a few hours, and check your filter. If it freezes again after thawing and replacing the filter, the issue is likely refrigerant-related and needs professional service.
Failing Compressor
The compressor is the heart of the AC system, it’s what actually creates the refrigeration cycle that cools your home. A failing compressor often manifests as an AC that runs but simply can’t keep up, even when refrigerant levels and airflow are fine.
Compressor failures are more common in older systems, or in systems that have been running under stress for an extended period (low refrigerant, restricted airflow) without being addressed. A failed compressor is a major repair, depending on the age of your system, a full replacement may be more cost-effective.
Thermostat Issues
A malfunctioning thermostat might be reading the wrong temperature or failing to communicate properly with the AC unit, causing it to cycle incorrectly. If your system seems to run but never reaches the set temperature, and the other causes above have been ruled out, a technician can test the thermostat as part of a diagnostic.
Undersized System for Your Home
Less common, but worth mentioning: if an AC system was never properly sized for your home, it may struggle to cool effectively even when it’s working perfectly. This is more likely to show up on extremely hot days when the system is at full capacity but can’t keep up with the heat load. An HVAC technician can calculate whether your system is appropriately sized.
When to Call for Service Immediately
If your AC has stopped working completely rather than just under-cooling, or if you notice ice on the lines, a burning smell, or the outdoor unit is not running at all, those warrant a same-day call rather than troubleshooting.
If you’ve been hearing unusual sounds from your system, that’s a related problem worth investigating alongside the cooling issue.
Get the Right Diagnosis Before Spending Money
The worst thing you can do with an AC that isn’t cooling is throw parts and refrigerant at it without knowing the root cause. A proper diagnostic from a licensed technician will tell you exactly what’s wrong, what it’ll cost to fix, and whether repair or replacement makes more sense for your specific system.
Mission Air Conditioning provides honest AC diagnostics and repair for McAllen and Rio Grande Valley homeowners. We’ll tell you what we find and give you clear options — no guesswork, no upselling. Call Mission Air Conditioning today to schedule an AC diagnostic visit.