AC Not Blowing Cold Air: Causes and a Step-by-Step Diagnosis

What "not blowing cold" usually means

Your indoor blower and your outdoor unit are separate machines. The blower moves air across the indoor coil; the outdoor unit runs the compressor that makes that coil cold. When air comes out of the vents but is not cold, either the coil is not getting cold (outdoor unit problem or refrigerant problem) or the air is not crossing the coil properly (airflow problem, or an iced-over coil).

A healthy system usually cools the air passing through it by about 14–22 °F (8–12 °C). Air at the supply register is commonly in the mid to upper 50s °F when the house is around 75 °F. Measure yours with the AC vent temperature calculator; the result tells you whether you have a small shortfall or no cooling at all.

Likely causes, ranked by how often they appear

1. Thermostat settings. Mode set to heat or off, setpoint above room temperature, or the fan set to ON so that air keeps blowing between cooling cycles and feels warm.
2. Dirty air filter. Low airflow reduces capacity and can freeze the coil.
3. Outdoor unit not running. Tripped breaker, failed capacitor or contactor, or a tripped condensate float switch. See outside AC unit not running.
4. Frozen evaporator coil. Caused by low airflow, low refrigerant or running at very low outdoor temperatures. Once iced, the coil blocks air and cooling collapses.
5. Dirty outdoor coil. The system cannot shed heat, so capacity drops on hot days.
6. Low refrigerant from a leak. Systems do not use up refrigerant; a low charge always means a leak.
7. Leaking or disconnected ductwork. A disconnected supply duct in a hot attic dumps cold air outside the living space and can pull in hot air.
8. Weather beyond design. Most systems are designed to hold the indoor temperature about 20 °F below a design outdoor temperature. On a record hot day, warm-ish air at the vents may be normal.
9. Failing compressor. The outdoor fan runs but the compressor does not, or it runs without pumping.

Symptom guide

Diagnostic sequence, from the safest checks

1. Check the thermostat. Mode on COOL, setpoint at least 3 °F below room temperature, fan on AUTO, fresh batteries if it uses them. Smart thermostats can also block cooling through schedules, eco modes or a minimum outdoor temperature setting.
2. Replace the filter. Hold the old one up to a light; if you cannot see light through it, it was restricting airflow. The filter guide covers sizes and change intervals.
3. Check the breakers. Both the indoor unit and the outdoor unit have their own breakers. Reset a tripped one once; if it trips again, see AC breaker keeps tripping.
4. Look at the outdoor unit while cooling is called. The fan should spin and you should hear the compressor hum. If nothing is running, or only the fan runs, note it for the technician.
5. Look for ice. Check the insulated suction line at the outdoor unit and, if you can see it, the indoor coil. If iced, set the thermostat to OFF and the fan to ON for two to four hours to thaw it, with towels under the air handler for the melt water. Do not chip ice off the coil.
6. Clean the outdoor coil. With the power off at the disconnect, rinse the coil from the inside out with a garden hose and clear plants back to about 2 ft (60 cm).
7. Measure the temperature split. After 15 minutes of steady running, measure air at a return grille and at a nearby supply register. Under about 14 °F with good airflow points toward a refrigerant or compressor issue.
8. Technician checks. Refrigerant charge by superheat and subcooling, leak search, capacitor and contactor testing, compressor diagnosis and blower speed set-up.

Adding refrigerant without finding the leak is a short-term patch, and handling refrigerant legally requires EPA Section 608 certification. DIY recharge cans sold for cars are not suitable for home systems.

What repairs typically involve and cost

Typical US price ranges, including labour:

On R-22 systems installed before about 2010, refrigerant is expensive and a leak repair is often a cue to price a replacement instead. The refrigerant types guide explains why.

Preventing a repeat

• Replace filters every one to three months in cooling season.
• Rinse the outdoor coil each spring and keep plants trimmed back.
• Have the system serviced annually, with refrigerant pressures and electrical parts checked.
• Keep supply registers open; closing them reduces airflow over the coil.
• Set realistic expectations on extreme days. Holding 72 °F when it is 105 °F outside is beyond what most systems are sized for; see the ideal AC temperature guide.

Frequently asked questions