If your AC is running but not cooling, check four things first: the thermostat (set to COOL, fan on AUTO, a few degrees below room temperature), the air filter, the breakers, and the outdoor unit (running, with the fan spinning and the coil clear of debris). Those checks fix a lot of calls. If the outdoor unit won't run, the air stays warm, or you see ice, call a technician.
Not sure what's wrong?
Try our quick troubleshooter →Living in Fallbrook means mild winters and warm summers, and in our inland valleys the AC works hard from late spring into fall. Even well-maintained systems have problems. This guide updates our original 2024 post with the troubleshooting steps we'd give you over the phone, and clear lines on what to leave to a licensed technician.
Electrical safety. Your outdoor unit has high-voltage parts, including capacitors that can hold a charge after the power is off. Don't open panels on the indoor or outdoor unit. If you smell burning or see smoke, turn the system off at the thermostat and the breaker, and call us.
What should I check first when the AC isn't cooling?
Start with the thermostat, filter, breakers and outdoor unit. Make sure the thermostat is on COOL with a setpoint below room temperature, replace a dirty filter, check that both AC breakers are on, and look at the outdoor unit: it should be running, with the fan spinning and air blowing warm off the top.
- Thermostat. Set to COOL, fan on AUTO, a few degrees below the current temperature. Replace batteries if the screen is dim or blank.
- Air filter. Hold it up to a light. If light barely gets through, replace it. A clogged filter is the single most common cause of weak cooling.
- Breakers. The indoor unit (furnace or air handler) and the outdoor unit usually have separate breakers. Reset a tripped one once. If it trips again, leave it off and call.
- Outdoor disconnect. Near the outdoor unit there's a disconnect box. Make sure it hasn't been pulled or switched off.
- Outdoor unit. With the AC calling for cooling, the fan on top should spin and you should hear the compressor. Clear leaves, weeds and anything stacked against it.
- Vents. Open supply registers and make sure furniture or rugs aren't blocking returns.
- Condensate switch. Many systems have a float switch on the drain line or pan. If water has backed up, it shuts the AC off. See the water-leak section below.
Why is my AC blowing warm air?
If the indoor fan runs but the air is warm, the outdoor unit usually isn't doing its job. Common causes are a tripped outdoor breaker, a failed capacitor or contactor, a dead outdoor fan motor, or low refrigerant from a leak. Capacitors and contactors are among the most common summer failures in North County.
If the outdoor unit is silent or humming without the fan turning, turn the system off at the thermostat to protect the compressor, and call for repair. Those parts are electrical and need a technician.
| Symptom | Likely causes | Safe homeowner step | Call a technician if… |
|---|---|---|---|
| AC won't turn on | Thermostat setting or batteries, tripped breaker, float switch tripped by a backed-up drain | Check thermostat, breakers and drain pan | Breaker trips again, or nothing is obvious |
| Running, but air is warm | Outdoor unit off, failed capacitor or contactor, fan motor, low refrigerant | Check outdoor breaker and disconnect; clear debris | Outdoor unit is silent, humming, or fan won't spin |
| Weak airflow, rooms not cooling | Dirty filter, blocked vents, frozen coil, duct leaks | Replace filter; open vents | Airflow stays weak after a new filter |
| Ice on lines or indoor coil | Low airflow or low refrigerant | Turn cooling off, run fan ON to thaw; replace filter | It freezes again |
| Water around the indoor unit | Clogged condensate drain, cracked pan, frozen coil thawing | Turn AC off; wipe up water; check the drain | Water keeps coming back |
| Turns on and off often | Dirty filter, thermostat in sun or near heat, oversized system | Replace filter; shade or relocate heat sources near thermostat | Short-cycling continues |
| Squealing, grinding, banging | Motor bearings, loose or broken parts | Turn the system off | Always. Don't open the unit |
| Bills climbing | Dirty coils, duct leaks, aging equipment | Replace filter; rinse outdoor coil gently with power off | Bills stay high; ask for a tune-up |
Why is my AC frozen?
Ice forms on the indoor coil or refrigerant line when too little warm air crosses the coil, usually from a clogged filter or closed vents, or when refrigerant is low. Turn cooling off and set the fan to ON to thaw it, which can take a few hours. Replace the filter before restarting. If it ices again, call us.
Don't chip at the ice. The coil fins are thin and easily damaged.
Why is water leaking from my AC?
Your AC pulls moisture from the air and drains it through a condensate line. If algae or debris clogs that line, water backs up into the pan and can overflow, or trip a float switch that shuts the AC off. A frozen coil that thaws can also overflow the pan.
- Turn the AC off and wipe up standing water.
- Find where the drain line exits outside (often near the outdoor unit or a wall) and check it isn't blocked.
- If your system has a drain access port, clear it following the manufacturer's guidance, or ask us to show you during a tune-up.
- Once the water drains, the float switch usually resets and the AC can run again.
Is low refrigerant something I can fix?
No. An AC doesn't use up refrigerant. If it's low, there's a leak. Handling refrigerant requires EPA certification, and a technician should find and repair the leak, then charge the system to the right level. Older R-410A systems can still be serviced and repaired.
Signs of a leak include weak cooling, ice on the lines, a hissing sound, or oily spots on fittings. See our refrigerant leak repair page and our R-410A guide.
Why does my AC turn on and off so often?
Frequent cycling usually comes from a dirty filter, a thermostat getting false readings from sun or a nearby lamp or oven, or a system that's too big for the house. An oversized AC cools the air quickly and shuts off before it removes humidity, leaving rooms cool but clammy.
The first two you can fix: replace the filter, and keep heat sources away from the thermostat. Oversizing is a design issue. When you eventually replace, ask for a load calculation instead of matching the old unit's size.
What are the strange noises telling me?
Squealing often points to a motor bearing or belt; banging or clanking to a loose or broken part; rattling to a loose panel or debris; buzzing or humming without the fan turning to an electrical problem. Turn the system off and call. These repairs involve moving parts and high voltage inside the unit.
Why are my cooling bills going up?
Rising bills with similar use usually mean the system is working harder for the same cooling: dirty filters and coils, leaky ducts in a hot attic, low refrigerant, or simply an aging system. SDG&E electricity is expensive, so efficiency losses show up quickly on the bill.
A tune-up cleans and checks the parts that drift out of spec. If the ducts are the problem, sealing them properly (with mastic or approved tape, not ordinary cloth duct tape) is a job for a professional, and it's required with a system replacement under California's Energy Code. If the system itself is the problem, our repair or replace guide can help.
How do I prevent these problems?
Most of these problems trace back to airflow and neglected parts. Change your filter on schedule, keep the outdoor unit clear, and have the system checked each spring before the heat arrives. Our top 10 maintenance tips and a yearly tune-up cover the routine. If it's already too hot to wait, go straight to our AC repair page or call (760) 728-8716.
“Great job explaining the various problems. Very courteous knowledgeable guys”
Frequently asked questions
Why is my AC running but not cooling?
The most common causes are a dirty air filter, a dirty outdoor coil, a thermostat set wrong, a failed outdoor fan or capacitor, a frozen indoor coil, or low refrigerant from a leak. Start by checking the thermostat, filter and outdoor unit. If the outdoor unit isn't running or the air stays warm, call a technician.
Why is my AC frozen?
An indoor coil freezes when too little warm air moves across it, usually from a clogged filter or closed vents, or when refrigerant is low. Turn the cooling off and run the fan on ON to thaw the coil, which can take a few hours. Replace the filter. If it freezes again, call a technician.
Why won't my AC turn on at all?
Check that the thermostat is set to COOL and below room temperature, that its batteries are good, and that the AC breakers are on. Many systems also have a float switch that shuts the AC off when the condensate drain backs up. If none of those is the cause, the problem is likely electrical and needs a technician.
Can I add refrigerant to my AC myself?
No. Handling refrigerant requires EPA certification, and low refrigerant means there's a leak somewhere. Adding more without finding the leak wastes money and releases refrigerant into the air. A technician should find and repair the leak, then charge the system correctly. R-410A systems can still be serviced.