Quick answer
An AC that runs but doesn't cool has six causes ranked: dirty filter, frozen indoor coil (result of #1), dirty outdoor condenser, low refrigerant (leak — needs a pro), failed capacitor (pro replacement), or failed compressor (usually means new system). The first three are DIY — check in order. If none help, call an HVAC tech. Never add refrigerant yourself — EPA-regulated and requires certification.
Key facts
| Common causes | Dirty air filter; Frozen indoor coil; Dirty outdoor condenser; Low refrigerant (leak) |
|---|---|
| DIY difficulty | Moderate |
| Time to fix | 30 min |
| Parts cost | $0–$80 parts |
| Tools needed | Screwdriver, garden hose, level |
| When to call a pro | Refrigerant issues · capacitor · compressor |
What's happening
An AC works by circulating refrigerant between two coils: an indoor evaporator that absorbs heat from your air (making the air cold) and an outdoor condenser that dumps that heat outside. A compressor drives the refrigerant. Anything that restricts airflow across either coil, or reduces refrigerant charge, or fails a component in the cycle stops cooling. Ice on the indoor coil is a symptom of restricted airflow — usually a dirty filter. Warm air blowing from vents when the outdoor unit runs = restricted airflow, low refrigerant, or compressor issue.
Common causes (ranked by frequency)
- Dirty air filter 40% of cases
Restricts airflow across the indoor coil. Coil freezes. AC blows warm air (or nothing). Replace filter, wait for ice to melt (4–24 hours with system off).
- Frozen indoor coil 20%
Symptom of dirty filter, closed vents, or low refrigerant. Ice buildup blocks airflow. Turn off cooling, run fan only, wait for melt (up to 24 hours).
- Dirty outdoor condenser 15%
Leaves, grass clippings, and dust coat the condenser fins. Heat can't escape. Rinse with garden hose (never pressure washer) from inside out.
- Low refrigerant (leak) 15%
Refrigerant doesn't 'get used up' — if it's low, there's a leak. Requires HVAC tech to find leak, repair, and recharge. Not DIY — EPA-regulated.
- Failed capacitor 8%
Starts the compressor and fan motor. When it fails, outdoor unit hums but doesn't start. $30 part but replacement requires opening the disconnect and discharging the capacitor safely — pro job.
- Compressor failure 2%
The heart of the AC. When it fails, replacement is often more than a new system. For 15+ year old systems, replace the whole outdoor unit.
How to diagnose
- Feel the vents. Warm air or no air = airflow problem or refrigerant. Cool air but weak = airflow restriction.
- Check the indoor coil (behind the filter access, or above the furnace). Ice = frozen coil. Turn AC off, run fan only, wait 4–24 hours for melt.
- Check the filter. Replace if dirty. If already fresh, look at other airflow restrictions (closed vents, blocked returns).
- Look at the outdoor unit. Fan running? Coils clean? Debris blocking airflow?
- If all of the above check out and the AC still doesn't cool, low refrigerant or component failure — call an HVAC tech.
The fixes
Fix #1 — Replace filter and let coil thaw First
The most common cause and the simplest fix. But if the coil is frozen, you have to wait for it to fully melt before running cooling again.
Tools
Materials
- Replacement air filter (MERV 8–11)
- Turn AC off at the thermostat. Run fan only if the coil is frozen — helps melt.
- Replace the filter.
- Check that no vents are closed and no returns are blocked by furniture or rugs.
- Wait for full melt — up to 24 hours for heavy ice buildup. Running cooling with ice present causes compressor damage.
- Restore cooling and test.
Fix #2 — Clean the outdoor condenser
Debris on the outdoor coils dramatically reduces cooling capacity. 15-minute rinse restores it.
Tools
- Garden hose (spray nozzle)
- Screwdriver
- Shop vacuum (optional)
Materials
- Coil cleaner (optional, $10)
- Turn OFF power at the disconnect box on the wall near the outdoor unit AND at the breaker.
- Remove debris (leaves, twigs, grass clippings) from around the base.
- Rinse the coil fins from INSIDE out using a garden hose with normal water pressure. Never pressure wash — bends fins.
- Optional: Apply coil cleaner per instructions, let sit, rinse thoroughly.
- Straighten bent fins gently with a fin comb (optional, $5).
- Restore power. Wait 5 minutes before starting — the compressor needs time to equalize pressures.
Prevention
Replace filter monthly during cooling season. Rinse outdoor condenser twice per year (spring startup and mid-summer). Annual HVAC tune-up ($100–$200) verifies refrigerant charge, cleans coils, and catches capacitor weakness before it strands you. Never plant shrubs closer than 2 feet from the outdoor unit — they restrict airflow.
When to call a pro
When to stop and call a pro
- Filter, coil melt, and condenser cleaning done — still not cooling.
- Outdoor unit hums but the fan doesn't turn — capacitor failure, needs pro (potentially lethal discharge risk in capacitors).
- Ice reforms within hours despite good airflow — low refrigerant.
- Loud grinding, screeching, or knocking from outdoor unit — compressor failing, don't run.
- System is more than 12–15 years old and repair estimate exceeds $1,000 — consider replacement.
HVAC diagnostic call: $100–$200. Refrigerant leak repair + recharge: $500–$1,500. Capacitor replacement: $200–$400. Full AC replacement: $4,000–$9,000.
Cost estimate
Cost estimate
Frequently asked questions
No. Refrigerant handling requires EPA 608 certification, and it's illegal to purchase in bulk without it. More importantly — if refrigerant is low, there's a leak. Adding refrigerant without finding the leak means it'll be low again next month. Call a licensed HVAC tech.
15–20°F cooler is the industry standard. Less than 15°F suggests airflow issue or low refrigerant. Measure with an infrared thermometer or a regular thermometer taped to a vent.
Chronic freezing means chronic airflow restriction (dirty filter, blocked returns, undersized ductwork) OR chronic low refrigerant (persistent leak). Fix the root cause — repeated freezing damages the compressor.
Yes — modeling shows 10–15% AC energy savings from good schedule-based setback plus geofencing. Confirm your existing thermostat wiring includes a C wire; if not, budget for an installer or use a smart thermostat with battery/adapter.