9 Reasons Your Air Conditioner Starts Then Stops, and What to Check

What counts as short cycling

A normal cooling cycle runs long enough to cool the house and pull moisture out of the air. Lennox puts a normal cycle at about 10 to 20 minutes. Carrier says a healthy system runs for about 15 to 20 minutes, and that an AC which shuts off after only a few minutes, or cycles about every 10 minutes, may be short cycling.

Short runs alone do not prove a fault. Trane notes that a properly sized system may only run 3 to 5 minutes and stay off for hours in mild weather. The warning sign is a run that stops before the house reaches the set temperature, then starts again soon after.

Some pauses are built in. Rheem's manual for its split-system outdoor units says they wait 5 minutes before restarting after a pressure switch trips. Frigidaire's room air conditioner manual says the compressor shuts off when you change modes and restarts in about 3 minutes in COOL.

In energy saver (econ) mode, a Frigidaire window unit shuts the compressor off at the set temperature and cycles the fan to sample the room, which is normal. If a mode name on your remote is unclear, our guide to air conditioner modes and symbols explains each one.

Short cycling costs you in three ways, according to Carrier: higher energy use, a house that stays humid, and extra strain on the compressor and fan motors. The same checks apply if you have a heat pump in cooling mode. If yours runs but never cools, see our guide to a heat pump not cooling.

9 reasons why your air conditioner starts then stops

Work down this list in order. The first five are things you can check yourself with the power off where noted. The last four need a technician to confirm.

1. Thermostat placement or settings

A thermostat that reads the wrong temperature tells the AC to stop early. Trane says one in direct sunlight can turn the AC on regardless of the actual temperature, and that it belongs in a central location, away from registers and heat-generating appliances. Moving a badly placed thermostat can sometimes fix short cycling on its own.

Lennox says to mount it on an interior wall away from heat sources or direct airflow, and to check the batteries and settings. On a standard setup, Rheem's manual has you set the system switch to COOL or AUTO and the fan switch to AUTO. Carrier lists wiring problems as another way a thermostat sends erratic signals, and that check belongs to a technician.

On a window unit, the thermostat is a sensor behind the filter. Frigidaire's room air conditioner manual lists a temperature sensing element touching the cold coil as a reason the air does not feel cold enough. The fix it gives is to straighten the sensor away from the coil.

2. A clogged air filter

This is the easiest cause to fix. Carrier says a clogged filter restricts airflow, so the system overheats and shuts down as a safety precaution. Carrier and Trane both recommend replacing the filter every 1 to 3 months, and ENERGY STAR's maintenance checklist says to inspect it once a month.

Window unit manuals agree. Frigidaire's troubleshooting table lists "turns on and off rapidly" with a dirty filter as the first cause. Wash the filter in warm water with dish detergent, or vacuum it, and let it dry fully before you put it back.

3. A frozen evaporator coil

Ice on the indoor coil blocks airflow, and American Standard says frozen coils can make a cooling unit short cycle. Trane lists the signs: ice on the coil or its cabinet, frozen refrigerant lines at the outdoor unit, warm air from the vents and puddles as the ice melts.

What to do, from Trane and Carrier:

1. Switch the thermostat from COOL to OFF. Trane warns that running with a frozen coil can cause major damage.
2. Set the fan to ON so warm air moves across the coil, as Carrier's frozen AC guide advises.
3. Do not chip at the ice, which Trane says can damage the coil. Carrier also warns against hair dryers, space heaters and hot water.
4. Check the filter and replace it if it is dirty, and watch that the drain line is not overflowing as the ice melts.
5. Restart only once airflow is back to normal and everything is dry. Carrier says light frost can take 1 to 3 hours and heavy ice 4 to 8 hours or longer.

On a Frigidaire window unit, the manual says to set FAN ONLY mode to defrost the coil. Our step-by-step guide to unfreezing an air conditioner has more detail. If the coil freezes again, Carrier says it is time to call a professional, because low refrigerant or a faulty part may be behind it.

4. A blocked or dirty outdoor unit

The outdoor unit has to push heat out. Lennox says leaves, dirt and overgrowth restrict airflow and can cause overheating, and that if the compressor shuts off from overheating, you should wait 30 minutes before restarting it.

Turn the power off at the breaker first. Carrier says to clear leaves, branches and plants within a two-foot radius of the unit, and to rinse the coil with a gentle hose stream, never a pressure washer, which can crush the fins. Debris inside the cabinet is a job for a technician.

A blocked coil can also trip a safety switch. Rheem's outdoor unit manual describes a high-pressure switch that shuts the system off when pressure climbs too high, and lists poor air circulation through a dirty or blocked condenser coil as a cause of a pressure trip.

For window units, Frigidaire says to keep shrubs away from the outside of the unit and, in extreme heat, to set a faster fan speed. If the unit will not restart after a shutoff, our guide on how to reset your air conditioner walks through it.

5. A tripped condensate overflow switch

Many indoor units have a switch in the drain pan that stops the AC before water overflows. Trane says it shuts off the outdoor compressor, the condenser fan and the supply fan, and that a sudden shutoff in hot weather can point to a clogged condensate line or pan. Dirt, debris, mold growth and even spider webs cause the clog. Not every air handler has this switch.

Carrier gives the homeowner steps to clean an AC drain line: turn the AC off, find the access point near the indoor unit and the exit outside, then use a wet/dry vacuum at the exit or pour a cup of distilled vinegar into the access point.

Lennox says to make sure the float switch is not stuck, and that a faulty one may need replacing. Our page on the AC float switch explains how it works, and if the drain smells musty, see how to get rid of mold in an AC.

6. Low refrigerant

Low refrigerant usually means a leak. Lennox says the system cannot absorb enough heat, the pressure turns erratic and the unit shuts off early to protect itself. Ice on the evaporator coil can be a sign.

Rheem's manual describes a low-pressure switch on select models that shuts the outdoor unit off on a loss of charge, then waits 5 minutes before it can restart. That on, off, wait pattern is one way low refrigerant looks like short cycling. Do not add refrigerant yourself. Lennox says handling it requires EPA certification, so a licensed technician finds the leak and recharges the system.

7. An oversized unit

Trane says an oversized system reaches the set temperature too quickly and shuts off before the cycle is complete. It usually happens when an installer sizes by square footage alone, or swaps in the same size as the old unit without a load calculation. American Standard says the installer likely skipped a Manual J calculation, the standard method for sizing a home's cooling load.

The giveaway is a house that feels cool but clammy. ENERGY STAR explains that an oversized window unit cools the room before removing humidity. For central systems, Trane and Lennox say replacing the unit with a correctly sized one is the only lasting fix. For a single room, our room size calculator checks your unit against the ENERGY STAR sizing table.

8. A failing capacitor or contactor

The capacitor gives the compressor and fan motors the burst of energy they need to start. Carrier's capacitor guide says a failing one often struggles to keep the compressor running, so the unit shuts off and restarts rapidly. Other signs are a hum from the outdoor unit, weak cooling and a swollen or leaking capacitor.

The contactor is the switch that sends power to the compressor. Carrier's AC parts guide lists buzzing and intermittent operation as warning signs. This is technician work. Carrier warns that capacitors store high-voltage electricity that can cause serious injury or death even with the power disconnected, and recommends that homeowners do not replace one themselves.

9. The control board or a time-delay relay

Some outdoor units carry a diagnostic module. Rheem's manual lists flash code 3 on its Comfort Alert module as short cycling, with the compressor running only briefly. The causes it gives are an intermittent signal from the thermostat, a defective time-delay relay or control board, or a pressure switch. A technician tests the voltages behind those codes, so leave the cover on and note any flashing light for them.

When to call a pro

Call a technician if the AC still cycles about every 10 minutes or less after you have checked the thermostat, which is Carrier's line. Call sooner if the coil refreezes, you see ice or hear strange noises, or the breaker trips again after one reset, which Lennox says points to a deeper electrical problem.

Before the visit, Rheem's manual suggests confirming that the thermostat is set correctly and that power to the unit is on. Write down when the cycling happens and what you have already tried. You can find a local technician on our Find a Pro page.

How to prevent short cycling

• Check the filter monthly and replace it every 1 to 3 months.
• Keep plants and debris at least two feet from the outdoor unit.
• Keep indoor vents clear of furniture.
• Have the condensate drain line cleared at each service visit; Carrier says professional cleaning is the best way to prevent repeat clogs.
• Book a yearly tune-up; Lennox suggests an inspection every spring and fall.

A tune-up covers what you cannot check yourself. ENERGY STAR's list for a contractor includes the refrigerant level, cleaning both coils, the condensate drain, tightening electrical connections and checking that the system starts, runs and shuts off properly.