AC Running But Not Cooling? Start Here.

It's the middle of summer in Forney and your AC is running. The unit outside is humming, the air handler inside is blowing. But the house is 82 degrees and getting warmer.

This is one of the most common calls HVAC contractors receive in North Texas, and a significant portion of them turn out to be something the homeowner could have fixed themselves in five minutes. Working through this checklist before you call a contractor can save you a service fee and get your house cool faster.

This guide covers the most common causes in order of how easy they are to check, starting with the things that take 30 seconds.

Step 1: Check Your Thermostat Settings

This sounds obvious, but it solves the problem more often than anyone wants to admit.

Check that the thermostat is set to COOL, not HEAT or OFF. Check that the fan is set to AUTO, not ON. When the fan is set to ON, it runs continuously even when the system is not actively cooling, which means warm air is circulating through the house and the temperature reading on the thermostat climbs.

Set it to COOL and AUTO. Drop the set temperature 3 to 5 degrees below the current room temperature. Wait five minutes and confirm the compressor outside kicks on.

If the thermostat screen is blank or unresponsive, check the batteries. A dead thermostat battery will shut down the entire system.

Step 2: Check the Air Filter

A clogged air filter is the single most common reason an AC system runs but fails to cool effectively.

Your AC system pulls air through the filter before cooling it. When the filter is packed with dust and debris, airflow through the system drops dramatically. The evaporator coil gets too cold, freezes over, and can no longer transfer heat out of the air. The system runs continuously but moves almost no cool air into the house.

Locate your filter. It's typically in the return air vent (a large grille, usually on a wall or ceiling) or in the air handler itself. Pull it out and hold it up to a light. If you can't see light through it, it's overdue.

Replace it with a clean filter of the same size. If the system has been running with a clogged filter, the coil may already be frozen. Set the thermostat to FAN ONLY (not cooling) for 1 to 2 hours to let the coil thaw before running cooling again. Then switch back to COOL and see if airflow improves.

Filters should be replaced every 1 to 3 months in Texas, more often if you have pets or if the system runs heavily during summer months.

Step 3: Check the Circuit Breakers

Your HVAC system typically runs on two separate breakers:

one for the air handler inside and one for the compressor outside. If the outside unit is running but not cooling, the compressor breaker may have tripped.

Go to your electrical panel and look for any breakers in the tripped position (partway between ON and OFF, or flipped to OFF). HVAC breakers are typically labeled "AC," "HVAC," "Air Handler," or "Condenser."

To reset a tripped breaker: flip it fully to OFF first, then back to ON. Do not just push it from the middle position to ON without fully resetting it first.

If the breaker trips again within a few minutes of resetting it, stop there. A breaker that keeps tripping is protecting you from something -- an electrical fault, a short, or an overloaded component. Call a contractor at that point rather than forcing it.

Step 4: Check the Condensate Drain Line

Your AC system removes humidity from the air as it cools.

That moisture collects in a drain pan and exits through a condensate drain line, usually a PVC pipe that drains outside or into a floor drain.

When this line gets clogged with algae, mold, or debris, the drain pan fills up. Most modern systems have a float switch that shuts the system down when the pan fills, which prevents water damage but also stops your cooling.

Look for your drain pan, which sits beneath the air handler. If there's standing water in it, the line is likely clogged.

The fix: locate the condensate drain line (the PVC pipe exiting the air handler) and pour a cup of distilled white vinegar into the access point on the line. Let it sit for 30 minutes, then flush with water. On some systems you can also use a wet/dry vacuum to pull the clog out from the exterior end of the drain line.

If the pan is overflowing or there's visible water damage, turn the system off and call a contractor.

Step 5: Check the Outdoor Unit

Go outside and look at the condenser unit (the large box with the fan on top). Check for these things:

Ice or frost on the unit or the copper lines running into the house. Ice on the outside unit typically means the refrigerant is low or the system has a restricted airflow problem (often from the clogged filter covered in Step 2). Turn the system off and let it thaw for 2 to 3 hours before running it again. If ice forms again after thawing, you need a contractor.

The fan on top is not spinning. If the compressor is running (you can hear it humming) but the fan is not turning, the fan motor may have failed. Turn the system off. This is a contractor call.

The unit is buried in debris, leaves, or overgrown vegetation. The condenser needs clear airflow on all sides to reject heat. If it's surrounded by vegetation within 18 inches, that restricts airflow and reduces efficiency. Clear the area and hose down the coil fins gently from the outside in.

The unit is running non-stop and the house still won't cool. On a 100-degree Texas day, most residential AC systems are designed to maintain a 20-degree difference between inside and outside. If it's 104 outside, your system may not be able to get below 84 inside. This is normal operation, not a system failure. Shade the condenser if possible, close blinds on sun-facing windows, and wait for the temperature outside to drop.

Step 6: Check for a Frozen Evaporator Coil

The evaporator coil sits inside the air handler (or in the ductwork above it).

When airflow is restricted or refrigerant is low, the coil temperature drops below freezing and ice forms on it. A frozen coil blocks airflow almost completely.

Signs of a frozen coil: the system is blowing air but it's barely moving, or it feels warm. The copper refrigerant lines running into the air handler may be frosted or iced over. There may be water dripping around the air handler as ice melts.

To thaw a frozen coil: turn the system to FAN ONLY for 1 to 3 hours. Do not run cooling while the coil is frozen. Once thawed, check and replace the air filter, then restart cooling. If the coil freezes again, the issue is either a persistent airflow restriction or low refrigerant -- both require a contractor.

When to Stop Troubleshooting and Call a Contractor

Work through the steps above first. If none of them resolve the issue, or if you run into any of these situations, it's time to call:

- The breaker keeps tripping after reset
- There is ice on the outdoor unit after thawing and restarting
- The outdoor fan is not spinning
- There is water damage or the drain pan is overflowing
- The system is blowing warm air and the filter is clean
- The system is short-cycling (turning on and off rapidly in short intervals)
- It has been more than 2 to 3 years since a professional tune-up

Low refrigerant, failed capacitors, compressor issues, and dirty evaporator coils all require a licensed HVAC technician. These are not DIY repairs.

Not Sure What You're Dealing With?

If you've worked through the checklist and still can't figure out what's wrong, the HVAC troubleshooting tool on this site can help you narrow it down based on your specific symptoms.

Troubleshooting tool: https://forneyhvacpros.com/hvac-troubleshooting-tool

If you're ready to bring in a contractor, the free contractor matching tool matches you with a vetted local pro for your specific situation. Emergency same-day service, routine repairs, and full replacements -- the tool factors in urgency, system age, and what you're looking for to recommend the right contractor from the network.

Contractor matching tool: https://forneyhvacpros.com/find-a-quality-hvac-contractor

The service is free for homeowners. No obligation, no marketing calls.

Frequently Asked Questions

Why is my AC running but not cooling the house?
The most common causes are a clogged air filter, a frozen evaporator coil, a tripped circuit breaker, a clogged condensate drain line, or low refrigerant. Start with the filter -- it is the cause in a large share of service calls. Work through the full checklist above before calling a contractor.

Why is my AC not cooling below 75 (or 78, or 80) degrees?
On extremely hot days in North Texas, most residential AC systems are sized to maintain roughly a 20-degree difference between the indoor and outdoor temperature. When it's 100 degrees or above outside, the system may not be able to get below 80 inside. If the outdoor temperature is moderate and the system still can't reach the set temperature, a low refrigerant charge or an undersized system may be the cause.

How long does it take for a frozen AC coil to thaw?
A fully frozen evaporator coil typically takes 1 to 3 hours to thaw completely when the system is set to FAN ONLY. Do not run cooling mode during this time. Replace the filter before restarting cooling.

Is it bad to run my AC when it's not cooling?
Running the system continuously when it has a problem can make some issues worse. A system with a frozen coil running in cooling mode will not thaw the coil and may strain the compressor. If the system is not cooling after working through the checklist, turn it to FAN ONLY or OFF while you figure out the next step.

How do I know if my AC needs refrigerant?
You cannot check refrigerant level yourself. Signs of low refrigerant include the system blowing warm air with a clean filter, ice forming on the outdoor unit or copper lines, a hissing or bubbling sound near the unit, and consistently high electricity bills with reduced cooling output. A licensed HVAC technician must check and recharge refrigerant -- it is not a homeowner repair.

How much does an AC service call cost in Forney TX?
Most HVAC contractors in the Forney and Kaufman County area charge between $85 and $150 for a diagnostic service call. Repairs are additional and depend on what's found. Some contractors apply the diagnostic fee toward the repair cost if you proceed with the work. Ask before they arrive.