Frozen AC Coils: What to Do Right Now and Why It Keeps Happening

If you see ice on your AC unit, turn the system off right now, switch the fan to “on” (not “auto”), and let it thaw. That’s the immediate answer. The rest of this article explains why it happened and what a tech checks when we come out.

Turn It Off First

Running a frozen AC doesn’t just mean warm air, it can damage the compressor. Compressors are expensive. Turn the thermostat to “off” or switch to fan-only mode. You’re not fixing anything yet, you’re stopping the damage. Thawing takes anywhere from one to a few hours depending on how much ice built up.

Put some towels around the indoor unit and check the condensate drain pan, because melting ice has to go somewhere and sometimes it overflows.

Why It Froze: The Likely Causes

Ice on AC coils almost always comes down to one of two things: restricted airflow or low refrigerant. Everything else is less common.

Dirty or blocked air filter. This is the most common cause by a wide margin. When the filter is clogged, there’s not enough warm air moving across the evaporator coil. The coil gets too cold, moisture in the air freezes on contact. Check your filter first. If it’s gray and dense, replace it. This is a free diagnosis.

Low refrigerant (refrigerant leak). Refrigerant doesn’t “run out” under normal conditions, it leaks. When the charge drops, the pressure inside the coil drops too, and the coil temperature falls below freezing even on a warm day. Low refrigerant and a dirty filter can feel identical from the outside. You need a technician with gauges to tell them apart.

Blocked return vents or closed registers. Same principle as the filter. If half your registers are closed or a return vent is blocked by furniture, airflow drops and the coil can freeze. Walk through the house and make sure nothing is blocking the vents.

Dirty evaporator coil. Over time, dust and debris build up on the coil itself, reducing heat transfer. It’s slower to cause freezing than a dirty filter, but it happens. Cleaning it means opening the air handler and using coil cleaner, so that’s a service call, not a homeowner task.

Clogged condensate drain. A blocked drain line can cause water to back up and contribute to coil freezing. While the system thaws, look at the drain pan. If it’s full of standing water or the drain line isn’t dripping, mention that when you call, because it’s part of what we check.

Running the AC when it’s too cold outside. Most residential systems aren’t designed to run below about 60°F. If you’re running it on a cool Bay Area night, especially in the inland valleys where temps can drop, that can cause freezing even with a clean filter and proper charge.

After It Thaws: What to Check

Once the ice is gone, replace the filter if it needed it, clear any blocked vents, and run the system. Watch it for 30 minutes or so.

If it freezes again, or if it was blowing warm air even before it froze, the filter wasn’t the only issue. At that point you’re likely looking at a refrigerant leak or a dirty coil, and a technician needs to check the refrigerant pressures.

One thing worth noting: if you see ice on the refrigerant line outside (the larger insulated pipe going into the wall), that points more strongly to a refrigerant problem than just a dirty filter.

What You Can Do Before Calling

A few things are genuinely safe to handle yourself:

• Replace the air filter (and do it every 1 to 3 months going forward)
• Open any closed supply registers and move furniture away from return vents
• Let the system thaw completely before restarting it

That’s where safe homeowner territory ends. Anything involving the refrigerant circuit, the coil, the blower, or the condensate drain beyond a visual check needs a tech. Adding refrigerant without finding the leak just delays the real fix and costs more.

What We Check When We Come Out

The process is straightforward. We check the filter and static pressure to assess airflow restriction. We attach gauges to the refrigerant lines and read the pressures against the spec for that refrigerant type. If pressures are low, there’s a leak. We also check coil condition and blower motor speed while we’re at it.

A recharge without finding and fixing the leak is a temporary fix, the refrigerant just leaks out again. We find where it’s leaking, repair it, then recharge. On older systems using R-22 (banned from production and import since 2020), only reclaimed supply remains and the cost per pound has climbed sharply. Replacement often makes more sense than repair, and we’ll tell you honestly which way that math goes for your system.

When to Call

If your system refreezes after you’ve replaced the filter and cleared the vents, that’s your answer: call. Same if it was blowing warm air before it froze, or if you hear the compressor short-cycling (turning on and off every few minutes).

A refrigerant leak typically gets worse while you wait, not better. We’ll get you on the schedule fast, often same or next day when we can. If you want someone to put gauges on it and tell you exactly what’s going on, reach out at bayareahvacservice.com or call us directly at (925) 999-4095.

Key takeaways

• Turn the AC off immediately and switch the fan to 'on' to start thawing. Running it frozen can damage the compressor, which is a far costlier repair.
• A clogged air filter is the most common cause and the first thing to check. Replacing a dirty filter is the one thing you can safely handle yourself.
• If the system refreezes after a fresh filter, you likely have a refrigerant leak that needs a technician with gauges, not another filter.
• A clogged condensate drain can also contribute to coil freezing. Check the drain pan while the system thaws and mention it when you call.
• Refrigerant work requires EPA Section 608 certification. Adding refrigerant without finding the leak just delays the real fix and costs more.

Related questions

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 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

Further reading

• Mitsubishi Hyper-Heat (H2i): How It Keeps Putting Out Heat When It Gets Cold in the Bay Area , Most people think heat pumps quit when it gets cold. Mitsubishi's Hyper-Heat (H2i) line is built to keep working as temperatures drop, which matters more here than folks expect, especially on cold mornings and up in the East Bay hills.
• Mitsubishi Mini-Split Blinking a Fault Light? What It Means and What to Do , A blinking light on your Mitsubishi indoor head is the system telling you something is wrong. Here is what the blink pattern means, the checks you can safely do yourself, and when to bring in a factory-trained tech.
• Why Your Mitsubishi Mini-Split Is Leaking Water Inside (and How to Fix It) , A Mitsubishi mini-split dripping water inside almost always comes down to condensate that can't drain. Here's what causes it, the safe checks you can do yourself, and when to call a tech.