The 10-Minute Checks (Start Here)
1. Thermostat: mode, temperature, and batteries
Confirm it is set to COOL, not HEAT or FAN, and set at least 5 degrees below the current room temperature. If the display looks dim, replace the batteries. Dead batteries cause phantom failures that look exactly like a broken AC. Smart thermostat owners: check that it did not lose Wi-Fi and silently revert to an old schedule.
2. Air filter
Pull the filter out and hold it up to a light. If you cannot see light through it, replace it right now. A clogged filter is the single most common cause of poor cooling in residential systems. If the system has been struggling for a while, the evaporator coil may already be frozen. After replacing the filter, turn the AC off for 2 to 4 hours to let it thaw.
3. Breakers, indoor and outdoor
Check your electrical panel for a tripped breaker labeled AC, Furnace, or Air Handler. Then go outside to the disconnect box near the condenser. It has its own breaker or pull-out disconnect that trips independently of your main panel, and most homeowners do not know it exists. Reset it once. If it trips again immediately, stop and call a pro. That is an electrical fault, not a nuisance trip.
4. Outdoor unit: power, debris, and breathing room
Is the condenser fan spinning? If not, and the breakers are fine, the capacitor has likely failed. That is a $150 to $300 professional repair. Do not attempt it yourself unless you are qualified; capacitors store a charge that can injure you. Also clear leaves, grass clippings, and debris from around the unit. A condenser needs 2 feet of clearance on all sides to breathe. A unit buried in mulch and weeds cannot reject heat.
The 30-Minute Checks
5. Condensate drain line
A clogged drain line triggers a float switch that shuts the entire system off as a safety measure. Find the PVC drain line near your indoor unit and check the overflow pan for standing water. Clear the line with a wet/dry vacuum on the outside end, or pour a cup of white vinegar through it to dissolve the algae clog.
6. Frozen evaporator coil
Ice on the copper refrigerant lines or the indoor coil means the system is frozen solid. The usual causes are a dirty filter (which you just fixed), low refrigerant, or blocked supply vents. Switch to fan-only mode for 2 to 4 hours to thaw it. Never chip at the ice. One slip punctures the coil and turns a $200 service call into a $1,200 coil replacement.
7. Supply vents: open and unblocked
Walk through the house and check every supply vent. Each one should be fully open and not blocked by furniture, rugs, or curtains. Closing vents in "unused" rooms does not save energy. It raises static pressure, strains the blower, and actually makes cooling worse.
When to Stop and Call a Professional
- Ice keeps coming back after you replaced the filter. That points to a refrigerant leak, which is a pro-only repair.
- Hissing or bubbling sounds near the unit. That is refrigerant escaping. Federal law requires a licensed technician to handle it.
- Burning smell from the vents or unit. Turn the system off immediately and call. That is an electrical issue.
- The outdoor unit hums but the fan does not spin. Failed capacitor or motor. A quick fix for a tech, a dangerous one for you.
- The system is 12 or more years old and keeps breaking. At some point you are funding a replacement one repair at a time. See our replacement cost guide to run the math.
Prevent the Next Breakdown
Change your filter every 1 to 3 months, hose down the outdoor coil each spring, and schedule annual professional maintenance. A tune-up costs $75 to $200 and catches roughly 80 percent of failures before they strand you on the hottest day of the year. Find HVAC pros near you for tune-ups and repairs before summer hits.