This post overlaps with a fuller one: Why Your AC Freezes Up: Two Causes, One You Control.
Every cause of an AC freezing up comes down to one of two things: too little warm air moving across the indoor coil, or too little refrigerant inside it. The coil drops below roughly 32°F, the moisture it pulled from the air turns to ice, and the ice blocks even more airflow. Here are the five causes we see most often around Pasadena, CA.
The five causes
- Dirty filter. The most common one. Dust season here runs all summer, and a filter you cannot see light through is starving the coil.
- Closed or blocked vents. Shutting registers in spare rooms and putting furniture over floor vents cuts total airflow across the coil.
- Dirty indoor coil. Years of fiberglass filters or gaps around the filter coat the coil in dust. A new filter does not fix it; cleaning does.
- Weak blower. A failing capacitor or motor moves less air than the system needs.
- Low refrigerant. Refrigerant does not get used up. Low means a leak, and the freeze-up returns until the leak is repaired.
Running the AC on a cool night, or leaving the fan on "on" while the system struggles, makes any of these worse.
What to do right now
Turn cooling off and set the fan to "on" so house air thaws the coil over a few hours. Do not chip the ice. Replace the filter and open every vent while it thaws. If it freezes again with a clean filter, stop and call; a compressor running against a frozen coil does not last.
When it needs a tech
A second freeze-up, hissing at a refrigerant line, weak airflow with everything open, or a blower that will not start all need a diagnostic visit. Spring maintenance catches most of these before the first heat wave.
The full guide: Why Your AC Freezes Up: Two Causes, One You Control.
Book a $69 diagnostic at Boost Services at 818-277-5056 or send us a message. The fee is credited toward the work.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.