Why is my AC running but not cooling the house?

The most common causes are a dirty air filter restricting airflow, a failed run capacitor preventing proper compressor or fan operation, low refrigerant charge from a leak, a frozen evaporator coil, or a failing compressor. Start with the filter and capacitor as they are the easiest to diagnose and most frequently the cause.

How do I know if my AC capacitor is bad?

Signs of a bad capacitor include: the outdoor unit hums but the fan does not spin, the compressor runs but the condenser fan does not (or vice versa), the unit struggles to start and trips the breaker, or the capacitor appears visually swollen or bulging. Confirm with a capacitor meter - a reading more than 6% outside rated MFD means replace it.

Can I run my AC if the refrigerant is low?

Running an AC with low refrigerant stresses the compressor, which operates outside its design parameters without proper cooling and lubrication. Extended operation with low refrigerant can cause permanent compressor damage - the most expensive repair in HVAC. If you suspect low refrigerant, have a licensed technician diagnose and repair the leak before adding refrigerant.

Why does my AC freeze up?

AC systems freeze when the evaporator coil gets too cold, which happens from insufficient airflow (dirty filter, closed vents, low fan speed) or low refrigerant charge. The ice insulates the coil and eventually blocks all airflow. Turn the system off, let it thaw for 2-4 hours with the fan on, then diagnose the root cause before restarting.

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AC TROUBLESHOOTING

AC FREEZING UP

Ice forming on your AC coil or refrigerant lines means something is preventing normal heat exchange. Here is how to thaw it safely and find the root cause so it does not happen again.

?? Difficulty: Beginner to Intermediate

?? Tools: Multimeter, clamp meter

Stop - Do This First

? Do Not Run AC While Frozen

If your AC is frozen, turn it OFF immediately. Running a frozen system can destroy the compressor. Set your thermostat to Fan Only (not cooling) to thaw the ice faster. Allow 2-4 hours for complete thawing before diagnosing further.

Why AC Systems Freeze

Your evaporator coil operates just above freezing during normal operation - typically 35-40?F surface temperature. Two things cause it to drop below 32?F and ice over:

Root Cause	Mechanism	Symptoms
Insufficient Airflow	Not enough warm air passing over the coil - the coil stays too cold	Ice on coil and suction line, reduced airflow from vents, dirty filter
Low Refrigerant Charge	Too little refrigerant means lower suction pressure = lower coil temperature	Ice on coil, low suction pressure on gauges, reduced cooling capacity

Diagnosing After Thawing

Check the Air Filter - First Always

A severely clogged air filter is the single most common cause of a frozen AC. If the filter is gray and blocked with dust, you found your problem. Replace it, let the system run for a day, and monitor. Also check that all supply and return vents in the house are open and unobstructed - closing vents does not save energy, it raises static pressure and reduces airflow, causing the coil to freeze.

Check Blower Motor Speed and Operation

The blower must move enough CFM - typically 400 CFM per ton of cooling capacity - to keep the coil above freezing. Measure blower motor amperage with a clamp meter and compare to nameplate FLA. A motor drawing significantly less than FLA may be on a low-speed tap. Check the blower wheel for dirt buildup - a dirty wheel can reduce airflow by 30-40% while the motor appears to run normally. On ECM motors, verify the motor is running in cooling mode airflow, not a reduced heating speed.

Check Refrigerant Charge with Manifold Gauges

After confirming airflow is adequate, connect gauges. For R-410A with a fixed orifice metering device, calculate superheat: suction temperature minus saturation temperature at suction pressure. High superheat (above 15?F) combined with low suction pressure indicates undercharge. For TXV systems, subcooling at the liquid line is the primary charging indicator. Important: Refrigerant does not deplete normally - if the charge is low, there is a leak. Adding refrigerant without fixing the leak is temporary.

Check Evaporator Coil for Dirt Buildup

Even with a clean filter, the evaporator coil can accumulate a thin layer of dust and biological growth over years of operation. This insulates the coil surface, reducing heat transfer and causing the coil to run colder. Inspect the coil with a flashlight - if you see gray buildup between the fins, the coil needs professional cleaning. Do not use household cleaners on the coil - use HVAC-specific coil cleaner.

Parts That May Be Needed

Blower Motor (ECM or PSC)

If motor is drawing low current or running at wrong speed

Find Blower Motor

Run Capacitor for Blower

Failing capacitor causes blower to run slowly and inefficiently

Find Capacitor

Metering Device / TXV

A stuck-closed TXV causes evaporator freeze-up

Find TXV

Frequently Asked Questions

Why is my AC freezing up?

AC systems freeze when insufficient airflow (dirty filter, closed vents, slow blower) or low refrigerant charge allows the evaporator coil temperature to drop below 32?F. Moisture in the passing air then condenses and freezes on the coil, progressively blocking airflow until the system is ice-locked.

Can I run my AC if it is frozen?

No - running a frozen AC forces the compressor to operate with no load, which can cause overheating and permanent compressor damage. Turn the system completely off and run fan-only mode to thaw the coil before diagnosing and restarting.

How long does it take for a frozen AC coil to thaw?

A frozen coil typically takes 2-4 hours to thaw completely. Running the fan without cooling mode speeds the process. Have towels ready near the drain pan - the melting ice produces significant water that can overflow if the drain is partially clogged.

My AC keeps freezing every few weeks - what is wrong?

Recurring freeze-ups indicate a persistent root cause: a slow refrigerant leak, a partially blocked coil that is not fully cleaning with each thaw cycle, or a failing blower motor that gets worse as it heats up. If cleaning the filter did not permanently fix it, you need gauges to check refrigerant charge and a clamp meter to verify blower performance.

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