Your AC is running. You can hear the compressor outside. The fan is blowing. But the air coming out of the vents is barely cool, or not cool at all. This is one of the most common homeowner AC complaints, and it has a defined list of causes, almost all of which a licensed HVAC technician can diagnose and resolve in a single service visit.
Before calling for service, checking a few things yourself can save time and a service call fee. But most of the causes on this list require tools, training, and refrigerant handling certification to diagnose correctly. Homeowners who need a diagnostic service call can reach out to Air Conditioner Repair Thornton CO specialists at Roots HVAC for same-day or next-day service.
Here are the eight most common reasons an AC runs but does not cool, starting with the ones you can check yourself.
1. A Clogged or Dirty Air Filter
A filter that has not been changed in three to six months or more can become so restricted that airflow through the system drops to a fraction of its rated capacity. When the evaporator coil cannot get enough warm air moving across it, it cannot transfer heat efficiently. In severe cases, the coil surface temperature drops below freezing and the coil ices over, which blocks airflow almost completely and causes the system to blow room-temperature air from what little it can move around the ice.
Check this first: pull the filter and hold it up to the light. If you cannot see light through it, replace it before calling for service. A clean filter costs $5 to $25. If the coil has frozen, turn the system to fan-only mode for two to four hours to allow the ice to melt before restarting in cooling mode. If the problem returns after the filter change, there is another cause.
Filter replacement interval recommendation: every 30 to 90 days for standard 1-inch filters, every 6 to 12 months for 4 to 5-inch media filters.
2. Frozen Evaporator Coils
An evaporator coil freezes when it gets too cold, which happens when airflow across the coil is restricted (from a dirty filter, blocked return, or dirty coil surface) or when the refrigerant charge is low enough that the evaporator pressure drops below the freezing point of water vapor in the air.
A frozen coil is visible when you remove the access panel on the indoor air handler: the copper lines and coil surface will be coated in ice. Running the system in this condition damages the compressor because liquid refrigerant can return to the compressor from the frozen coil.
The fix depends on the cause. If the filter and airflow are the issue, the coil will usually defrost and operate normally after the filter is replaced. If low refrigerant is the cause, a technician needs to measure the charge, find the leak, repair it, and recharge the system.
Service call for coil defrost diagnosis: $100 to $200.
Refrigerant leak repair and recharge: $300 to $900, depending on the leak location and the amount of refrigerant needed.
3. Low or Leaking Refrigerant
Refrigerant is the working fluid that absorbs heat from indoor air and rejects it outside. A system with a refrigerant charge below specification cannot absorb the rated amount of heat, which means the air leaving the supply registers is warmer than it should be. The system runs longer trying to reach the set temperature, draws more power, and stresses the compressor operating in low-charge conditions.
Refrigerant does not deplete in a normal system. If the charge is low, there is a leak somewhere in the refrigerant circuit. Finding and repairing the leak before recharging is the correct procedure, not simply topping off the charge without addressing the source. A system recharged without leak repair will lose refrigerant again and need another recharge service call.
Leak detection: $75 to $150.
Repair: $150 to $600, depending on location.
Refrigerant recharge: $100 to $150 per pound for current refrigerants.
4. A Dirty Condenser Coil
The condenser coil in the outdoor unit rejects the heat that was absorbed inside. A coil covered in cottonwood seeds, grass clippings, pet hair, or dust film cannot transfer heat to the outdoor air efficiently. Head pressure inside the system rises, the compressor works harder, and the system’s overall cooling capacity falls.
You can visually inspect the condenser coil fins on the outside of the outdoor unit. If the fin surface is visibly clogged or if the fins are bent, the coil needs professional cleaning and/or fin combing. Professional condenser coil cleaning costs $75 to $200 and is typically included in an annual maintenance visit.
Do not spray the condenser coil from the outside with a garden hose at high pressure. This pushes debris deeper into the coil and can bend the fins. Cleaning from the inside out with a coil cleaner and low-pressure rinse is the correct method.
5. A Failed Capacitor
The run capacitor provides the electrical boost that keeps the compressor motor and condenser fan motor running at their rated efficiency. When a capacitor weakens or fails, the motor it serves runs at reduced power, draws excess current, generates excess heat, and may stop running entirely.
A system with a failed condenser fan capacitor will have the compressor running while the condenser fan sits still, which quickly causes the system to overheat and shut down on high-pressure limit. A system with a failing compressor capacitor may still start, but will draw excessive amperage and deliver reduced cooling capacity.
Capacitor failure is one of the most common AC service calls in summer heat. Capacitors weaken faster when the system runs in high ambient temperatures with reduced cooling on the capacitor itself from a dirty condenser coil or restricted airflow.
Capacitor replacement: $150 to $300 for the part and service call.
6. Thermostat Problems
Before assuming the cooling system is at fault, check that the thermostat is set correctly, reading temperature accurately, and communicating with the air handler and outdoor unit.
Common thermostat issues that cause poor cooling:
- Fan set to “ON” rather than “AUTO”: in the ON position, the blower runs continuously, including when the cooling system is not active. The result is room-temperature air blowing from the registers between cooling cycles, which feels like the system is not cooling.
- Thermostat sensor drift: a thermostat sensor that reads indoor temperature 3 to 5 degrees higher or lower than the actual temperature will cycle the system incorrectly. A simple temperature comparison using an accurate thermometer near the thermostat reveals sensor drift.
- Dead thermostat batteries: Many digital thermostats continue to display a screen even with low batteries, but lose the ability to communicate with the HVAC system reliably. Replace batteries annually.
Thermostat recalibration or replacement: $75 to $250, depending on the thermostat model.
7. Duct Leaks Losing Conditioned Air
An air conditioner can be working perfectly and still fail to cool the home if the conditioned air it produces is leaking into an unconditioned attic, crawl space, or wall cavity before it reaches the living space. The Department of Energy estimates that duct leakage accounts for 20% to 30% of cooling energy loss in a typical home.
Signs of duct leakage:
- Rooms at the end of long duct runs that never reach the set temperature
- Supply registers that deliver weak airflow even when the system is running at full capacity
- A home that has significantly higher indoor humidity than expected, given outdoor conditions
Duct leakage diagnosis requires a blower door test or duct pressure test. Sealing leaky duct connections costs $300 to $700 for a contractor to address the accessible joints. Full duct sealing with Aeroseal or similar technology runs $1,500 to $3,000 but typically reduces duct leakage to near zero.
8. An Oversized System That Short-Cycles
An air conditioner that is too large for the home reaches set temperature too quickly, shuts off before completing a full dehumidification cycle, and restarts shortly after. The result is a home that cools to a set temperature but feels clammy and uncomfortable because the system has not run long enough to remove adequate moisture from the air. On very hot days, a dramatically oversized system may also overshoot the set temperature in one zone while not reaching it in another.
Short-cycling also causes rapid cooling at the thermostat sensor while the rest of the home is still warm, creating a false reading that triggers shutdown before the whole home is comfortable.
An oversized system cannot be fixed by repair. The solution is correct equipment sizing using a Manual J load calculation, followed by replacement if the installed capacity is significantly above the calculated load. ACCA data shows that HVAC equipment oversizing is present in approximately 50 to 70 percent of residential installations. If your system has never cooled the home comfortably despite otherwise clean maintenance and functional components, oversizing may be the root cause.
