AC running but not cooling, the most common culprits are a dirty air filter, incorrect thermostat settings, dirty condenser coils, or low refrigerant. Start with the filter and thermostat — both are free to check. If those aren’t it, turn the system off and call a licensed technician before a small problem becomes a compressor replacement.
It’s the scenario every homeowner dreads during an August heat wave in Snohomish County: the system hums along, air comes out of the vents — but it’s not cold. Before you assume the worst, work through these eight causes. The first three are DIY-friendly. The rest need professional tools.
1. Dirty Air Filter
A clogged filter chokes airflow across the evaporator coil, which can cause weak, lukewarm air — or a frozen coil. If you can’t remember the last time you changed it, that’s your answer. During heavy summer use, check it monthly.
2. Thermostat Settings or Dead Batteries
Confirm the thermostat is set to COOL and the fan to AUTO (not ON — that blows uncooled air between cycles). Swap in fresh batteries and make sure the set temperature is actually below the room temperature.
3. Dirty or Blocked Outdoor Condenser
Your outdoor unit dumps heat outside — it can’t do that job wrapped in cottonwood fluff, grass clippings, or overgrown shrubs. Clear 2 feet of space around the unit and gently rinse the coil fins with a garden hose (power off first, never a pressure washer).
4. Low Refrigerant (a Leak)
Refrigerant doesn’t get “used up” — if it’s low, there’s a leak. Signs include hissing sounds, ice on the copper lines, and gradually weaker cooling. This is sealed-system work that legally requires an EPA-certified technician.
⚠ Pacific Northwest Note
After our long, damp winters, the first hot stretch of August is when weak capacitors and slow refrigerant leaks finally show themselves. If your system struggled during last summer’s heat waves too, don’t wait for a full breakdown — repairs are faster and cheaper before the peak-season rush.
5. Frozen Evaporator Coil
Ice on the indoor coil blocks heat transfer entirely. It’s usually caused by restricted airflow (see #1) or low refrigerant (see #4). Turn the system off and let it thaw completely — running it frozen can destroy the compressor.
6. Closed or Blocked Supply Vents
Walk the house. Furniture, rugs, and “helpfully” closed vents in unused rooms raise static pressure and reduce cooling everywhere. Keep at least 80% of your vents open.
7. Failed Capacitor or Compressor
If the outdoor fan isn’t spinning, or you hear clicking as the unit tries to start, a failed capacitor is likely — a quick, inexpensive fix for a pro. A failed compressor is the expensive version of the same symptom, and often the moment replacement makes more sense than repair.
8. An Undersized or Aging System
If your AC runs nonstop and never quite catches up on hot afternoons, it may simply be undersized for your home — or past its 12–15 year prime. A load calculation during an AC replacement consultation will tell you for sure.
5–15%
of AC energy use can be saved just by replacing a clogged filter — the cheapest “repair” in HVAC.
Frequently Asked Questions
Should I turn my AC off if it’s not cooling?
Yes — if you see ice, hear hissing or grinding, or the outdoor fan isn’t spinning, turn it off at the thermostat and call a technician. Running a malfunctioning system can turn a minor repair into a major one.
How much does AC repair cost in Snohomish County?
Simple fixes like capacitors or contactors typically run a few hundred dollars. Refrigerant leak repairs and compressor work cost more. We always diagnose first and quote flat-rate before any work begins.
Can maintenance prevent this?
Most of the eight causes above are caught during a routine tune-up. Our AC maintenance service includes coil cleaning, refrigerant checks, and electrical testing that catch problems before a heat wave does.
AC Not Cooling? We’ll Find It Fast.
Glacier Heating & Cooling serves Snohomish and King County with same-week AC diagnostics and flat-rate repairs.Call (425) 422-8861
