Why is my AC not cooling?

An AC stops cooling when 1 of 5 common causes fails: a thermostat set to heat, a clogged filter limiting airflow, a refrigerant leak, a failed capacitor or a clogged condensate drain line.

Key takeaways

Check the basics first

thermostat settings, the air filter, the breaker and the drain line are the only do-it-yourself checks.

Identify the symptom

short cycling, warm air, ice, water leaks, thermostat faults and a failed capacitor each point to a different cause.

Stop the AC when ice forms

turning off the AC and running the fan thaws the evaporator coil.

Call a technician in 6 situations

refrigerant leaks, returning ice, a burning smell, a tripping breaker, water inside the home and no cooling after the basic checks.

Why does my AC turn on and off?

Short cycling is an AC shutting off before the AC finishes a cooling cycle. There are 3 common causes of short cycling.

Oversized system

An AC with a tonnage larger than the cooling load of the home cools the home quickly and shuts off before a full cycle. Tonnage sets the run time; the sizing guide What Size AC Does a Home Need? explains how the size is set.

Thermostat placement and settings

A thermostat near a heat source, in direct sun or in a draft reads a wrong temperature and ends the cycle early. The temperature differential setting also sets how often the AC cycles.

Dirty condenser or restricted airflow

A dirty condenser coil or a restricted airflow path raises the pressure in the system, and a safety limit then shuts the AC off. Cleaning the coil and replacing the filter restore the airflow.

Why is my AC blowing warm air?

An AC blows warm air when the system fails to remove heat from the air. There are 4 common causes: a thermostat set to heat or fan-only, a dirty air filter or blocked condenser, low refrigerant and a failed compressor.

Thermostat set to heat or fan-only

A thermostat set to heat or to a constant fan blows unconditioned air. Do-it-yourself steps are limited to non-hazardous checks: thermostat settings, the air filter, the breaker and the drain line. Refrigerant, gas and electrical panel work belongs to a licensed technician. Set the thermostat to cool and lower the temperature below the room temperature. Confirm the fan setting reads auto.

Dirty air filter and blocked condenser

A clogged filter or a condenser covered by debris restricts airflow, and the coil then fails to release or absorb heat. Check the air filter and replace it, if it is clogged.

Low refrigerant

Refrigerant does not get used up, so a leak is the only cause of low refrigerant. Refrigerant leak detection and repair is licensed work; the refrigerant rules are in the section on the R-410A phase-out below.

Failed compressor

Warm air with the outdoor unit silent or humming points to a failed compressor. A compressor failure is a technician repair.

Why is my AC freezing up?

Ice forms on the evaporator coil when airflow or refrigerant drops below what the coil needs. There are 3 common causes of a frozen AC.

Restricted airflow

A dirty filter, closed vents or a collapsed duct reduce the air moving across the evaporator coil, and the coil temperature falls below freezing.

Low refrigerant charge

A low refrigerant charge lowers the pressure in the coil, and the coil temperature falls below freezing. A refrigerant leak repair is licensed work.

Dirty evaporator coil

Dirt on the evaporator coil insulates the coil and blocks airflow, and ice forms on the coil. A technician cleans the coil.

Steps for a frozen AC

A frozen AC has 3 steps:

01
Turn off the AC at the thermostat.
Turning off the cooling stops ice from forming further.
02
Run the fan.
Running the fan alone moves room air across the coil and thaws the ice.
03
Call a technician, if the ice returns.
Returning ice points to a refrigerant or coil problem. Do-it-yourself steps are limited to non-hazardous checks: thermostat settings, the air filter, the breaker and the drain line. Refrigerant, gas and electrical panel work belongs to a licensed technician.

Why is my AC leaking water?

Water leaks from an AC when condensation fails to drain. There are 3 common causes of an AC water leak.

Clogged condensate drain line

Algae and debris build up in the condensate drain line and block the flow, and the water backs up and spills.

Cracked or overflowing drain pan

A cracked drain pan leaks, and an overflowing drain pan spills when the drain line is blocked.

Melting ice from a frozen coil

A frozen coil releases water as the ice melts, and the water exceeds the capacity of the drain pan. The causes of ice are in the section on a freezing AC above.

Why is my thermostat not working?

A thermostat stops working from power loss, wiring faults or sensor errors. There are 3 common causes.

Dead batteries or lost power

A thermostat with dead batteries or no power shows a blank display. Do-it-yourself steps are limited to non-hazardous checks: thermostat settings, the air filter, the breaker and the drain line. Refrigerant, gas and electrical panel work belongs to a licensed technician. Replace the batteries. Check the breaker for the AC circuit.

Wiring and compatibility faults

A loose wire or a thermostat that does not match the HVAC system stops the AC from starting. A technician checks the wiring and the compatibility.

Sensor placement and calibration

A thermostat sensor in direct sun, near a vent or on an exterior wall reads a wrong temperature. A sensor that reads off by several degrees needs calibration or relocation.

What does an AC capacitor do?

An AC capacitor stores and releases the electrical charge that starts and runs the compressor and fan motors. There are 2 things to know about a capacitor: the types and the signs of failure.

Start and run capacitors

A start capacitor gives a motor a short burst of charge at startup, for example the compressor. A run capacitor supplies a steady charge while the motor runs, for example the condenser fan.

Signs of a failed capacitor

A failed capacitor shows 4 signs:

Humming from the outdoor unit with a fan or compressor that does not start.
Clicking from the outdoor unit when the AC tries to start.
Warm air from the vents while the indoor fan runs.
Delayed startup of the outdoor unit after the thermostat calls for cooling.

A technician replaces a capacitor, because a capacitor holds a stored charge after the power is off.

What is the R-410A phase-out?

According to U.S. Environmental Protection Agency research from the Office of Air and Radiation, August 28, 2026, new residential and light commercial air conditioning and heat pump equipment manufactured or imported on or after January 1, 2025 is restricted to refrigerants with a global warming potential below 700. R-410A is a hydrofluorocarbon refrigerant that is being replaced in new equipment by lower-warming refrigerants. There are 2 effects of the phase-out.

Effect on repairs

The restriction applies to the manufacture, import and installation of new equipment. Existing R-410A systems remain in service, and a licensed technician continues to repair the systems.

Effect on replacement

New equipment uses the new lower-warming refrigerant, and the refrigerant type is a line on the quote. The replacement decision is covered in When Should You Replace an AC Unit?

When should a San Diego resident call an AC technician?

A San Diego resident calls a technician in 6 situations. The AC repair page AC Repair in San Diego, CA describes the repair visit.

Call for refrigerant leaks

Refrigerant handling is licensed work.

Call when ice returns after thawing

Returning ice points to a refrigerant or coil problem.

Stop the AC at an electrical burning smell

A burning smell points to an electrical fault, and a technician inspects the unit.

Call when the breaker trips repeatedly

A repeatedly tripping breaker points to an electrical fault in the AC.

Stop the AC when water enters the home

Water inside the home calls for shutting off the AC until a technician fixes the drain.

Call when the AC does not cool after the basic checks

Basic checks cover the thermostat, the filter, the breaker and the drain line.

Schedule San Diego AC Repair Service Today

No matter the HVAC issue, Sandiego HVAC Services has the solution for San Diego residents. Call +1 (858) 392-9754 or send the service request form with name, phone, ZIP or neighborhood and system type. See also AC Repair in San Diego, CA, 24-Hour Emergency HVAC in San Diego and AC Installation and Replacement in San Diego.