Why Is My AC Freezing Up? Five Causes and What to Do First

If your AC is running but not cooling, and you can see ice on the indoor coil or the copper refrigerant line outside, the system has a freezing problem. This is the single most common AC service call we get from May through August. The good news: most causes are straightforward to fix. The bad news: running a frozen system damages the compressor, so the diagnostic is time-sensitive.

First step: turn cooling off, fan on

Before anything else, switch the thermostat to OFF on cooling and turn the fan setting to ON. The blower keeps moving room-temperature air across the frozen indoor coil and thaws it safely. Running cooling while the coil is iced up pulls liquid refrigerant back to the compressor instead of vapor; that wears the compressor fast and is the most expensive failure mode on the system.

Wait 1 to 4 hours for full thaw, depending on ice volume. Set a towel under the indoor unit; melted ice can be a couple of gallons.

Five most common causes

1. Airflow restriction (most common)

Anything that blocks air across the evaporator coil drops coil temperature below 32°F. The usual suspects:

• Dirty air filter. Check first. If it’s gray or you can’t see light through it, replace it. New filter, no other problems, system runs fine again.
• Closed or blocked registers. Closing too many registers in unused rooms restricts return airflow. Open them.
• Collapsed flex duct. Common in attics where ducts kink against framing. Visible inspection finds these.
• Dirty evaporator coil. Coils accumulate dust over years and reduce airflow. Cleaning runs $200 to $400.
• Failed blower motor or capacitor. Motor running slow won’t move enough air. Diagnostic confirms it.

2. Low refrigerant

Refrigerant doesn’t get consumed in normal operation. If pressure is low, there’s a leak. Lower pressure drops the coil’s saturation temperature below freezing.

We use Fieldpiece gauges to measure suction and liquid line pressures, calculate superheat and subcooling, and confirm the leak. Common leak locations: evaporator coil seams, line set fittings, condenser coil end-cap brazing, Schrader valve cores.

Repair cost depends on refrigerant type. R-410A leaks: $400 to $1,200 for repair + recharge. R-454B (current standard for new installs): similar. R-22 (phased out): $1,000+ just for refrigerant; usually a replacement conversation.

3. Dirty evaporator coil

The indoor coil collects dust and biological film over years. Even a half-millimeter buildup measurably restricts airflow and lowers heat transfer. We clean the coil with a foaming non-acid cleaner; the system regains airflow and pressure.

Service tip: maintenance plans catch coil buildup before it becomes a freeze problem. A spring AC tune-up that includes coil cleaning typically prevents this from happening.

4. Low ambient temperature

Bay Area shoulder-season weather (April–May and October–November) can run AC in 55 to 65°F outdoor temps. Most ACs aren’t designed for that. Outdoor temp below the system’s low-pressure cut-out point lets coil temperature drop below freezing even with normal refrigerant charge.

If your AC freezes only on cool evenings and runs fine on hot days, this is your cause. Fix: thermostat with low-temp lockout, or just don’t run cooling below 65°F outdoor temp. Heat pumps with proper controls don’t have this problem because they cycle differently in shoulder season.

5. Failed blower motor or capacitor

Blower motors that are about to fail run slower than rated. Slower blower = less airflow = frozen coil. The simple test: hand against a register on cooling — if airflow feels weak compared to last summer, the motor is on the way out.

Capacitor degradation is the most common cause behind a slow blower. Replacement is $150 to $300. Full motor replacement runs $400 to $700.

What we do on a freeze call

A typical diagnostic visit:

1. Confirm system is OFF and coil is thawing (or thawed).
2. Inspect filter, registers, accessible ducts.
3. Inspect indoor coil for dust buildup.
4. Connect manifold gauges to refrigerant ports; read suction and liquid pressures, calculate superheat/subcool.
5. Check blower motor amperage and capacitor capacitance against spec.
6. Identify root cause(s); some calls have more than one (dirty filter AND low refrigerant is common).
7. Written estimate before any repair work.

Diagnostic is $75, credited toward any repair over $200.

When freezing means replacement

Three cases where we pivot to replacement conversation:

• System is on R-22 refrigerant and has a leak. Reclaimed R-22 cost makes repair uneconomical.
• System is past 15 years and the freeze is the third or fourth significant repair in a row.
• Indoor coil is cracked beyond cleaning, and replacement coil cost approaches half of a new system.

For all three we run the math at the estimate. We don’t push replacement when repair makes sense; we don’t quote a $400 repair on a system that will fail again next summer.

Related reading

• Repair or replace your HVAC — how I decide on the job
• Heat pump installation cost in the Bay Area
• Heat pump or gas furnace — 2026 decision guide