If you’ve spotted ice on your AC’s indoor unit, refrigerant lines, or outdoor condenser — or your system is running but not cooling — there’s a good chance your evaporator coil is freezing. Here’s what causes it and how to fix it.
Why Does an Air Conditioner Freeze Up?
Your evaporator coil works by absorbing heat from the air passing over it. For this process to work, two things must be true: enough warm air must flow over the coil, and the refrigerant inside must be at the correct pressure and temperature. When either condition is violated, the coil drops below 32°F and frost — then ice — forms on it.
Cause 1: Dirty or Blocked Air Filter
This is the #1 cause of AC freeze-ups in Kansas City. A clogged filter restricts airflow over the coil. Without enough warm air flowing across it, the coil gets too cold and freezes. Kansas City’s cottonwood season in May–June is particularly brutal on filters.
Fix: Turn the AC off. Switch the fan to “fan only” to let the coil thaw (takes 2–12 hours). Replace the filter. Turn the AC back on and monitor.
Cause 2: Low Refrigerant (Leak)
When refrigerant charge is low, the evaporator coil pressure drops, causing the coil to get colder than it should. A frozen coil shows up with ice on the indoor unit and often on the copper lines running between indoor and outdoor units.
Fix: This requires a licensed technician. They’ll find and repair the leak, then recharge the system. Do not just “top off” without fixing the leak — refrigerant doesn’t escape naturally.
Cause 3: Blocked or Closed Registers
Closing multiple supply or return registers restricts total system airflow. Make sure all registers in the home are open, even in rooms you don’t use often. If you’re closing registers to “save energy,” it’s actually doing the opposite — and potentially freezing your coil.
Cause 4: Dirty Evaporator Coil
Over time, especially with inadequate filtration, dust and debris build up on the evaporator coil. This acts as insulation, reducing heat transfer and causing the coil to freeze. A dirty coil requires professional cleaning.
Cause 5: Failing Blower Motor
If the blower motor is weak or failing, it can’t move enough air over the coil. Symptoms include weak airflow from vents (even though the AC is running) along with ice formation.
What to Do Right Now If Your AC Is Frozen
- Turn the AC off at the thermostat (switch to “Off”)
- Leave the fan running on “Fan On” to thaw the coil — or turn everything off and wait
- Check and replace the air filter
- Check that all registers are open
- Once fully thawed (2–12 hours), turn AC back on and monitor for 24 hours
If it freezes again after thawing and replacing the filter, the problem is refrigerant or mechanical — call a technician.
When Is a Frozen AC an Emergency?
On a 95°F Kansas City summer day with elderly residents, young children, or pets in the home, a non-functional AC is an urgent situation. Our network dispatches same-day technicians across the KC metro even for freeze-up issues. Submit a request here.
Written & Reviewed By
HVAC Repair KC
All articles are written or reviewed by licensed HVAC technicians serving the KC metro. Content is updated as codes, pricing, and equipment evolve.