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troubleshooting · · 5 min read

Why Is My AC Freezing Up? Five Causes and When to Call

If your AC is icing over the coil or refrigerant lines, one of five common causes is almost always responsible. We diagnose and repair frozen AC systems across Danville, San Ramon, and the Tri-Valley.

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Why Is My AC Freezing Up? Five Causes and When to Call

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 a common AC service call we get from May through August. Running a frozen system damages the compressor, so don’t ignore it. We diagnose the fault on site, then provide a written repair estimate and timeline.

First step: turn cooling off

Before anything else, switch the thermostat from Cool to Off. Running cooling with ice on the coil can damage the compressor, the most expensive part of the system. Let the ice melt on its own and watch the floor under the indoor unit, because a heavily iced coil sheds a lot of water and the drain pan can overflow.

How long it takes depends on how much ice built up. When you call, tell us where you saw the ice and whether any water leaked.

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 this first. If it’s gray or you can’t see light through it, replace it. A clean filter sometimes resolves the whole thing.
  • 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. A tech spots these on inspection.
  • Dirty evaporator coil. Coils accumulate dust and film over years, cutting airflow noticeably. Cleaning requires a foaming cleaner and safe access; it’s a pro job.
  • Failed blower motor or capacitor. A motor running below rated speed won’t move enough air. This needs electrical diagnostic and the right parts.

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.

A tech connects manifold gauges to the refrigerant ports, reads suction and liquid line pressures, calculates superheat and subcooling, and pinpoints the leak. Common leak locations: evaporator coil seams, line set fittings, condenser coil end-cap brazing, Schrader valve cores. Finding and fixing a refrigerant leak requires EPA 608 certification and recovery equipment; it’s not a DIY job.

Repair cost depends on the refrigerant type and where the leak is, and we quote it after the diagnosis. On R-22 (phased out), the refrigerant alone often makes repair uneconomical.

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. Coil cleaning uses a foaming non-acid cleaner and needs proper access; when it’s done right, the system regains normal airflow and pressure.

A spring AC tune-up that includes coil cleaning typically catches this before it causes a freeze.

4. Low ambient temperature

Bay Area shoulder-season weather (April to May and October to November) can put outdoor temps in the 55 to 65°F range. Most ACs aren’t designed for that. If your AC freezes only on cool evenings and runs fine on hot days, this is likely your cause.

The fix is a thermostat with a low-temp lockout, or simply not running cooling below 65°F outdoor temp. Heat pumps with proper controls handle this better 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 means less airflow means frozen coil. If airflow from the registers feels noticeably weaker than last summer, that’s a warning sign. A capacitor or motor swap is quick work for a tech with the right parts and a safe electrical shutdown; it’s not something to attempt without training.

What we do on a freeze call

A typical diagnostic visit:

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

Diagnostic is $99, credited toward any repair.

When freezing means replacement

Three cases where we’ll walk through replacement instead of repair:

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

We run the numbers at the estimate. We don’t push replacement when repair makes sense, and we won’t quote a repair on a system that’ll fail again next summer.

Call us

If your coil is iced up and the filter isn’t the problem, call us at (925) 999-4095. We cover Danville, San Ramon, Alamo, and the surrounding Tri-Valley and East Bay. Same or next-day service most days; we’ll tell you the window when we book it. Diagnostic is $99, credited toward the repair. CSLB #1136642.

Key takeaways

  • Frozen AC coil is almost always one of five causes: airflow restriction, low refrigerant, dirty coil, low ambient temperature, or blower failure.
  • Turn cooling off when you find ice at the indoor coil. Report any leaking water and arrange diagnosis before treating a thaw as a repair.
  • Running a frozen system damages the compressor; do not keep cooling on while ice is visible.
  • Most fixes are routine repairs. We price the repair after the diagnosis.
  • If your system is on R-22 refrigerant and has a leak, replacement usually wins on lifetime cost.

Related questions

Should I keep running my AC if it's iced up?
No. Turn cooling off and keep it off until the ice is gone. Tell us about any water leaks when you book. Thawing time varies, and melted ice doesn't mean the cause is fixed.

What's the most common cause of AC freezing up?
Restricted airflow. A clogged filter, closed registers, collapsed duct, or dirty evaporator coil all reduce air across the coil enough to drop coil temperature below freezing. Check the filter first and replace it if it's gray or clogged. Open any closed registers. If the system still freezes after that, call us. Dirty coils, collapsed ducts, and blower issues need a tech.

Could it be low refrigerant?
Yes, that's cause #2. Low refrigerant means lower evaporator pressure, which drops coil temperature below freezing. Refrigerant doesn't get used up; if the level is low, there's a leak. We pressure-test, find the leak, and quote either repair or replacement based on system age and refrigerant type. R-22 systems with a leak usually push toward replacement because R-22 is phased out and only reclaimed supply remains.

Can outdoor temperature cause AC freezing?
Yes, in mild Bay Area conditions especially. AC running on a 60°F night will freeze the coil because the refrigerant cycle isn't designed for that low ambient temperature. Most modern thermostats have a low-temperature lockout; older units sometimes don't. If your AC freezes only in early spring or fall evenings, this is likely the cause. We can set up a thermostat with proper low-temp lockout; in the meantime, don't run cooling below 65°F outdoor temp.

How much does it cost to fix a frozen AC?
A new filter runs $15 to $30 at a hardware store and is worth checking before you call us. Our on-site diagnosis is $99, credited toward the repair, and the written repair price follows the visit. R-22 systems with a leak often warrant a replacement conversation rather than repair.

Need help with your system? Call (925) 999-4095 or book a visit. Repair diagnostics are $99, credited toward the repair. Installation assessments are free.

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA Section 608 Universal certification, and is factory-trained on Daikin (Houston, 2025) and Mitsubishi. He writes from direct field experience, not marketing copy.

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