Your air conditioner is running, the fan is blowing, but the house keeps getting warmer. In an Arizona summer that’s more than an annoyance — it can become a safety issue fast. Here are the most common reasons an AC runs without cooling, what you can safely check yourself, and when it’s time to call a professional.

Quick answer

The most common causes are a thermostat setting problem, a dirty air filter, a tripped breaker or blown disconnect fuse, a dirty or blocked outdoor unit, a frozen evaporator coil, low refrigerant from a leak, a failed capacitor, or leaky ductwork. The first four you can often check yourself; the rest need a licensed HVAC technician.

Start with these safe checks

1. Thermostat settings

Make sure the thermostat is set to Cool, the fan is set to Auto (not On), and the set temperature is below the current room temperature. If the display is blank or dim, replace the batteries. A fan set to “On” will keep blowing room-temperature air even when the compressor isn’t cooling.

2. A dirty air filter

A clogged filter restricts airflow, which reduces cooling and can cause the indoor coil to freeze. Pull the filter and hold it up to a light — if you can’t see through it, replace it. Arizona dust means filters often need changing more often than the package suggests.

3. Tripped breaker or outdoor disconnect

If the indoor fan runs but the outdoor unit is silent, check your electrical panel for a tripped AC breaker. Reset it once. If it trips again, leave it off and call a technician — repeated tripping points to an electrical or compressor problem.

4. Outdoor unit blocked or dirty

The outdoor condenser has to release heat from your home. Clear away weeds, leaves and stored items so there’s at least a couple of feet of open space around it. Dust and debris on the fins also reduce performance; a professional cleaning restores it safely.

Problems that need a professional

5. Frozen evaporator coil

If you see ice on the refrigerant lines or water around the indoor unit, turn the system to Off and set the fan to On to help it thaw. Ice usually means an airflow problem or low refrigerant, and running a frozen system can damage the compressor.

6. Low refrigerant (a leak)

Air conditioners don’t “use up” refrigerant. If the level is low, there’s a leak that needs to be found and repaired — simply adding refrigerant without fixing the leak is a temporary fix at best.

7. Failed capacitor or contactor

These electrical parts help start the compressor and outdoor fan. A failed capacitor is one of the most common summer breakdowns: you may hear a hum from the outdoor unit while the fan doesn’t spin. Capacitors store electrical charge and should only be handled by a technician.

8. Leaky or disconnected ductwork

If some rooms are much warmer than others, cooled air may be escaping into the attic through torn or disconnected ducts. In Arizona attics that can waste a lot of cooling.

When to call right away

• The breaker trips again after one reset
• You see ice on the lines or the indoor coil
• You hear grinding, screeching or loud buzzing
• You smell burning or the outdoor unit hums but the fan won’t spin
• Indoor temperatures are climbing and someone at home is elderly, very young or has health concerns

Prevent it next summer

Many no-cooling calls trace back to issues a tune-up can catch early — weak capacitors, dirty coils and airflow problems. See what’s included in our HVAC tune-up, or learn about the SmartCare+ maintenance plan. Have more questions? Visit our HVAC FAQ.