Your AC is humming away, but your house feels like a sauna. This frustrating situation typically means something is blocking your system from delivering cold air, even though the unit itself is operating.
Step-by-Step Fixes
Step 1: Check Your Thermostat Settings
Head straight to your thermostat. Make sure it’s set to “Cool” mode, not “Fan Only” or “Heat.” This sounds obvious, but you’d be surprised how often this gets accidentally switched. Set the temperature at least 5 degrees below your current room temperature. If you have a programmable or smart thermostat, verify the schedule hasn’t been altered.
Step 2: Inspect and Replace Your Air Filter
Pull out your air filter right now. If it looks like a gray carpet of dust, that’s your problem. A clogged filter blocks airflow, making your AC run constantly without cooling effectively. Replace it with a new one – most homes need a new filter every 1-3 months. Standard sizes like 16x25x1 or 20x20x1 are available at any hardware store. This fix takes under 5 minutes and costs less than $20.
Step 3: Check All Air Vents
Walk through your house and ensure every supply vent is open and unblocked. Move furniture, curtains, or rugs that might be covering vents. Also check your return air grilles – these larger vents that suck air back into the system. If more than 20% of your vents are closed or blocked, your system can’t circulate properly.
Step 4: Examine the Outdoor Unit
Go outside and look at your condenser unit. Is it covered in leaves, grass clippings, or debris? Turn off the AC at the breaker, then gently clean the unit with a garden hose (not a pressure washer). Make sure there’s at least 2 feet of clear space around all sides. Bent fins on the unit can also restrict airflow – you can carefully straighten them with a fin comb from the hardware store.
Step 5: Check for Ice on the Indoor Coil
Turn off your AC and open the access panel near your indoor unit (usually in the attic, basement, or closet). If you see ice on the evaporator coil, you’ve found the issue. Turn the system to “Fan Only” mode to help melt the ice faster. This usually takes 2-4 hours. Once melted, you can restart the AC, but ice formation indicates a deeper problem that needs professional attention.
Step 6: Test Your Circuit Breakers
Check your electrical panel for any tripped breakers. Your AC typically has two breakers – one for the indoor unit and one for the outdoor unit. If a breaker is in the middle position, flip it fully off, then back on. A repeatedly tripping breaker indicates an electrical issue requiring professional help.
Likely Causes
Cause #1: Refrigerant Leak
Your AC uses refrigerant (often R-410A in units made after 2010) to absorb heat from your home. When refrigerant levels drop due to a leak, the system can’t cool effectively. Signs include hissing sounds, ice on the refrigerant lines, or higher than normal electric bills. You cannot add refrigerant yourself – this requires EPA certification. A technician will need to find the leak, repair it, and recharge the system.
Cause #2: Failed Capacitor
The capacitor gives your compressor and fan motors the electrical boost they need to start. When it fails, you might hear the unit humming but not fully engaging. The outdoor fan might not spin, or spin slowly. Capacitors are silver cylindrical components that can bulge or leak when failing. While technically replaceable by homeowners, the high voltage involved makes this best suited for professionals.
Cause #3: Dirty Evaporator or Condenser Coils
Over time, your AC coils accumulate dirt, reducing their ability to transfer heat. The evaporator coil (inside) can grow mold or collect dust, while the condenser coil (outside) gets caked with dirt and debris. Professional cleaning every 1-2 years keeps these components working efficiently. Neglected coils force your system to work harder, eventually leading to complete cooling failure.
When to Call Expert Help
Call an HVAC professional immediately if you notice any of these situations: electrical burning smells, repeated breaker trips, visible refrigerant leaks (oily spots on lines), or if basic troubleshooting doesn’t restore cooling within 4 hours. Most reputable companies offer same-day emergency service.
During peak summer months in 2025, technicians stay extremely busy. When calling, mention specific symptoms you’ve observed and steps you’ve already tried. This helps them bring the right parts and tools. Expect to pay $75-150 for a diagnostic visit, with repairs ranging from $200 for simple fixes to $2,000+ for major component replacements.
Copy-Paste Prompt for AI Help
“My air conditioner is running but not cooling my house. The unit turns on and I can hear it running, but the air coming from vents isn’t cold. I’ve checked that the thermostat is set to cool mode and replaced the air filter. The outdoor unit fan is spinning. What other troubleshooting steps should I try before calling a technician? Please provide specific, step-by-step instructions ideal for a homeowner with basic DIY skills.”