AC Not Cooling: Step-by-Step Checks for Central Air

An air conditioner that runs without cooling is working hard and getting nowhere. The cause can be as simple as a setting or a clogged filter, or as serious as a refrigerant leak or failed compressor. Working through the checks below in order separates the two quickly, and the first several need no tools.

How central air cools

The indoor coil, mounted on top of or beside the furnace, gets cold as refrigerant evaporates inside it. Warm house air blown across it gives up heat and moisture. The refrigerant then carries that heat to the outdoor condenser, where a fan blows air across the outdoor coil to release it. If indoor air cannot move across the indoor coil, or the outdoor unit cannot shed heat, cooling drops off.

Diagnose the symptom

What you notice	Likely cause
Indoor blower runs, outdoor unit silent	Tripped outdoor breaker or disconnect, failed capacitor or contactor
Outdoor unit runs, fan on top not spinning	Failed fan motor or capacitor
Weak airflow from vents	Dirty filter, blocked return, frozen indoor coil
Ice on refrigerant lines or indoor coil	Airflow problem or low refrigerant
Air is cool but not cold, unit runs constantly	Dirty outdoor coil, low refrigerant, undersized system on a very hot day, leaky ducts
Some rooms cool, others hot	Duct or balancing problem

Check 1: Thermostat

Set the mode to COOL, the fan to AUTO, and the temperature a few degrees below the room temperature. Replace batteries if the screen is dim. Many systems have a built-in delay of several minutes before the outdoor unit starts, so give it time.

Check 2: Filter and vents

A clogged filter is one of the most common reasons for poor cooling. ENERGY STAR advises inspecting filters monthly and notes that airflow problems can reduce a system’s efficiency by up to 15 percent. Replace a dirty filter and open any closed supply registers.

Check 3: Power to both units

Central AC usually has two breakers: one for the indoor furnace or air handler and a larger double breaker for the outdoor unit. There is also an outdoor disconnect box mounted near the condenser. Check that the breakers are on and the disconnect is in place. If the outdoor breaker trips again when reset, leave it off and call a technician.

Check 4: The outdoor unit

Look at the condenser while the system is calling for cooling:

• The fan on top should be spinning and the compressor should hum.
• Grass, leaves, cottonwood fluff and dirt on the coil fins block airflow. Clear debris and keep shrubs trimmed back. Our guide to cleaning an AC condenser covers rinsing the coil safely.
• ENERGY STAR notes that dirty coils reduce a system’s ability to cool and make it run longer.

If the fan is not spinning while the compressor hums, turn the system off at the thermostat to avoid damaging the compressor.

Check 5: Look for ice

Ice on the copper line at the outdoor unit or on the indoor coil means the coil is too cold, usually from poor airflow or low refrigerant. Turn the system to OFF and the fan to ON to thaw it, and read our guide to an AC freezing up.

Measure the temperature drop

With the system running for 15 minutes, hold a thermometer at a return grille and then at a supply register near the air handler. A properly working system cools the air noticeably as it passes the coil. If there is little difference, the system is not removing heat, which points toward refrigerant or compressor issues.

Refrigerant is a technician’s job

ENERGY STAR’s checklist calls for a contractor to check refrigerant level, because too much or too little refrigerant makes a system less efficient and shortens equipment life. Working with refrigerant requires EPA Section 608 certification, and a low charge means there is a leak that should be found and repaired, not just topped off.

Common mistakes

• Setting the thermostat very low to cool faster; it does not speed cooling and can freeze the coil.
• Covering the outdoor unit in summer.
• Hosing the condenser with power on, or bending the coil fins with high pressure.
• Buying DIY refrigerant kits for a system that requires certified handling.

When to call a technician

Call an HVAC technician if the outdoor unit will not run, the outdoor fan does not spin, the outdoor breaker trips, ice keeps forming after a filter change, or the air stays warm after the basic checks. ENERGY STAR also points to leaky ducts as a common cause of rooms that will not cool; in a typical house, about 20 to 30 percent of the air moving through ducts is lost to leaks and poor connections.