Most common causes
- Dirty air filter. This is the most common cause. Without enough warm air passing over the coil, the coil drops below freezing and moisture turns to ice.
- Closed or blocked vents. Closing many supply vents or blocking return grilles with furniture cuts airflow the same way a dirty filter does.
- Dirty indoor coil. Dust buildup on the evaporator coil insulates it and blocks air, so the coil runs colder than it should.
- Blower motor problem. A weak or failing blower motor, or a bad blower capacitor, moves too little air across the coil.
- Low refrigerant from a leak. When the charge is low, pressure in the coil drops and so does its temperature. Ice often shows up first on the larger copper line at the outdoor unit.
- Running in cool weather. Running the AC on cool nights, especially with a low setpoint, can let the coil freeze because the return air is already cold.
What you can safely check
- Set the thermostat to Off for cooling and switch the fan to On. Let it run 2 to 4 hours, or until all ice is gone. Do not chip or scrape the ice.
- Put towels down near the indoor unit. Melting ice makes a lot of water fast.
- Replace the air filter while the system is thawing.
- Open all supply vents and make sure return grilles are not blocked.
- Once fully thawed, turn cooling back on and check it over the next few hours. If ice comes back, turn it off and call.
Call a technician if
- Ice returns within a day after thawing with a clean filter
- The indoor fan does not run when set to On
- Water from melting ice is overflowing the drain pan or leaking through a ceiling
- You hear hissing or bubbling near the coil or lines
- The system froze and now the outdoor unit will not start
What affects the repair cost
If the cause is a dirty filter, the fix costs only a new filter. A dirty coil needs a cleaning, and blower motor or capacitor issues are parts and labor that depend on the motor type. A refrigerant leak is the most involved repair because the leak has to be found and fixed before recharging, and the cost depends on leak location and refrigerant type.
