AC Not Working? How to Diagnose It Before You Call

Work through these in order. The first four are free and solve most cases.

Symptom to cause

Start with the free three: thermostat set to cool and batteries good, breaker on, filter clean. Then check the outdoor unit is spinning. Ice anywhere means shut it off and let it thaw before anything else — running an iced system damages the compressor.

1. Thermostat

• Mode set to COOL, setpoint below room temperature.
• Fan on AUTO, not ON. Fan ON runs the blower continuously, circulating uncooled air between cycles — which feels exactly like the AC has stopped working.
• Batteries, if applicable.
• After a power outage: many systems impose a compressor restart delay of several minutes. Wait before diagnosing.

2. Air filter

The highest-value free check, and the root cause behind the frozen-coil problem below.

A restricted filter reduces airflow across the evaporator coil. Less air means less heat picked up, the coil gets colder than designed, and eventually it freezes — at which point cooling stops entirely.

Hold it up to a light. If light doesn't pass through, replace it. See HVAC air filters and MERV explained.

3. Breakers and disconnect

A central system typically has two circuits: outdoor unit and indoor air handler. Both need to be on.

Reset a tripped breaker by pushing it fully to OFF first, then ON. See how to reset a tripped breaker.

Also check the disconnect near the outdoor unit — a switch or pull-out in a small box on the wall. These get switched off for yard work and forgotten.

If a breaker trips again immediately, stop — that's a fault. See why does my breaker keep tripping.

4. Is the coil frozen?

The check people skip, and the reason a system sometimes seems to "fix itself" overnight.

Look for: ice on the refrigerant lines near the indoor unit, ice visible on the evaporator coil, water pooling around the air handler as it thaws, or air barely moving at the vents.

What to do:

1. Turn cooling OFF, set fan to ON. Moving air across the coil thaws it far faster.
2. Wait one to three hours.
3. Replace the filter while you wait.
4. Restart cooling and watch whether it refreezes.

Why it happened — two causes only:

• Insufficient airflow — dirty filter, blocked returns, closed vents, a dirty blower wheel, or a failing blower motor
• Low refrigerant charge, which means a leak

If it refreezes after a filter change, it's the second one. See refrigerant leak signs.

Never run the compressor with a frozen coil. Liquid refrigerant reaching the compressor is how compressors die.

5. The outdoor unit

Is it running? Fan spinning, compressor humming.

Is it dirty? The condenser coil rejects heat to outdoor air. Caked with cottonwood, grass clippings, or dust, it can't. This is a very common summer failure and it's user-serviceable: power off at the disconnect, rinse the coil gently with a hose from the inside out. Not a pressure washer — it bends fins.

Is it clear? Vegetation, fencing, or stored items restricting airflow. Trim back to manufacturer clearance.

Fan running but compressor silent? Likely a failed capacitor or contactor — see below.

6. Capacitor — technician territory

The start/run capacitor gives the compressor and fan motor the kick they need to start. It's one of the most common failure parts in air conditioning, and it fails more often in extreme heat.

Symptoms: a humming or buzzing outdoor unit that doesn't start; the fan runs but the compressor doesn't; a clicking that repeats without the unit starting; sometimes a visibly bulged or leaking capacitor.

It's a relatively inexpensive part. Capacitors store a dangerous charge even with power off and must be discharged properly — this is not a homeowner repair despite what videos suggest.

7. Refrigerant — technician only

Low charge reduces capacity and freezes coils. Refrigerant is not consumed, so low charge means a leak.

Signs: ice on the lines, long run times with weak output, hissing or bubbling, oily residue near joints, and a system that worked fine last summer.

A top-up without a leak search is a recurring bill, and running low on charge damages the compressor. See refrigerant leak signs.

8. Condensate drain shutoff

Many systems have a float switch in the condensate pan that cuts the system off if water backs up — deliberately, to prevent water damage.

Symptom: the system simply won't run, with no other fault evident. Look for standing water in the drain pan or at the air handler.

Clearing the drain line usually resolves it. This is a common late-summer call and an easy one to check yourself.

When it cools but can't keep up

Different problem from "not cooling". If the air at the vents is genuinely cold but the house never reaches setpoint:

• Extreme outdoor temperature. Systems are sized to a design temperature; beyond it they run continuously by design.
• Duct losses. Ducts in a hot attic lose a substantial share of their cooling before it reaches a room. See ductwork problems and cost.
• Insulation and air sealing. The load may exceed the equipment.
• An aging system that has lost capacity.
• Undersized equipment, though this is far less common than oversized — see what size air conditioner do I need.

If it's cold but clammy rather than not cold, that's a humidity problem and usually an oversizing symptom — see home humidity control.

What to have ready for a technician

• Is the outdoor unit running? Fan? Compressor?
• Air temperature at a supply vent versus a return vent
• Any ice, and where
• Filter last changed when
• When it last worked normally, and what changed

Where to go next

• Air conditioner maintenance checklist
• Refrigerant leak signs
• How long does an air conditioner last
• AC short cycling causes
• Run the numbers: electricity cost calculator

More in our heating and cooling guides.