What to Do When Your AC Stops Working in the Heat

Step 1: Check the Thermostat

Your thermostat is the brain of your AC system, and misconfigured settings cause more "emergency" calls than any actual mechanical failure.

• Confirm the mode is set to "Cool" (not "Heat," "Auto," or "Off")
• Set the temperature at least 5 degrees below the current room temperature
• If the screen is blank, replace the batteries — dead batteries are a common cause of system shutdown
• If you have a smart thermostat, check the app for error codes or schedule overrides
• Try switching to "Fan Only" mode — if the fan runs but cold air doesn't come, the issue is likely the compressor or refrigerant, not the thermostat

If the thermostat is working correctly, move to Step 2.

Step 2: Check the Circuit Breaker

Your AC system uses two breakers — one for the indoor air handler and one for the outdoor condenser. Either can trip independently.

1. Find your electrical panel (usually in the garage, basement, or a utility closet)
2. Look for breakers labeled "AC," "HVAC," "Air Handler," or "Condenser"
3. If a breaker is in the middle position (between ON and OFF), it's tripped
4. Flip it fully to OFF, wait 30 seconds, then flip back to ON
5. Wait 3–5 minutes for the system to restart — most AC units have a built-in delay

Important: If the breaker trips again immediately, do not keep resetting it. This indicates an electrical fault — a short, a grounded compressor, or a wiring issue that needs a licensed electrician or HVAC technician. Repeatedly resetting a tripping breaker is a fire risk.

Step 3: Check and Replace the Air Filter

A severely clogged air filter restricts airflow so much that the evaporator coil freezes over, shutting down the entire system. This is one of the most common and cheapest AC problems to fix.

1. Locate your air filter (usually behind a return vent on the wall or ceiling, or inside the air handler unit)
2. Pull the filter out and hold it up to light — if you can't see light through it, it's clogged
3. Replace it with the correct size (printed on the filter frame) — standard filters cost $5–$15 at any hardware store
4. After replacing, turn the system off for 2–4 hours to let any ice on the coil melt before restarting

In humid cities like Houston and Miami, filters clog faster due to moisture and higher particulate loads. Replace every 30–60 days during summer, not the 90 days printed on the package.

Step 4: Check the Outdoor Unit

Your condenser unit (the big box outside) needs clear airflow to dump heat. Go outside and inspect it:

• Clear debris — remove leaves, grass clippings, or anything within 2 feet of the unit
• Check if the fan is spinning — if the outdoor fan isn't running but you can hear a hum, the fan motor or capacitor may have failed
• Look for ice — frost on the refrigerant lines or the unit itself usually means low refrigerant or a frozen coil
• Listen for unusual sounds — grinding, screeching, or loud buzzing indicates a mechanical failure that needs a pro

In Phoenix and Dallas: Check if the condenser is in direct afternoon sun. Desert and Texas heat can push condenser operating temperatures past their limits. A shade structure (that doesn't restrict airflow) can improve performance by 5–10%.

Step 5: Stay Safe While You Wait for Repair

If the troubleshooting steps above didn't fix the problem, you need professional help. Here's how to stay safe until the technician arrives:

Manage indoor temperature:

• Close all blinds and curtains — direct sunlight can raise room temperature 10–15 degrees
• Open windows on opposite sides of the house to create cross-ventilation (only if outdoor temperature is below indoor temperature)
• Use ceiling fans and portable fans — they don't cool air, but the wind chill effect makes it feel 4–6 degrees cooler
• The CDC warns: if indoor temperature exceeds 90 degrees, fans alone are not enough and can actually increase body temperature

Cool your body directly:

• Drink cold water frequently — don't wait until you're thirsty
• Apply cold wet towels to your neck, wrists, and forehead
• Take cool showers or baths
• Place bowls of ice in front of fans for a makeshift evaporative cooler

Reduce internal heat:

• Don't use the oven or stove — eat cold meals
• Turn off all unnecessary lights and electronics (they generate heat)
• Avoid running the dishwasher or dryer

Protect vulnerable people: Move children under 5, adults over 65, and anyone with heart conditions, diabetes, or respiratory issues to a cooled location — a neighbor's house, a public library, or a cooling center. Call 211 to find cooling centers in your area.

Step 6: Call for Professional Repair

When you call an HVAC company, have this information ready to speed up diagnosis:

• Your system's make and model (on a label on the indoor or outdoor unit)
• What happened — did it stop suddenly, gradually lose cooling, or make unusual noises?
• What you've already tried — thermostat, breaker, filter
• Your system's age — this helps them bring likely parts

What to expect on cost:

• Diagnostic service call: $75–$200
• Emergency/after-hours surcharge: $100–$250 on top of the service call
• After-hours labor rates: $160–$250/hour vs. $75–$150/hour during business hours
• Common repairs: $150–$600 (capacitor, contactor, thermostat)
• Major repairs: $750–$2,500 (compressor, coil replacement)

Tip: Many HVAC companies waive the diagnostic fee if you proceed with the repair. Ask before booking.

What NOT to Do When Your AC Fails

Don't keep turning the system on and off. Rapid cycling (turning the AC on and off repeatedly) can damage the compressor — the most expensive component in your system ($750–$2,500 to replace). If it doesn't start after one attempt, leave it off.

Don't pour water on the outdoor unit. While some homeowners try this to cool the condenser, it can cause electrical shorts and void your warranty.

Don't ignore a tripping breaker. If the breaker trips more than once, something is electrically wrong. Forcing it back on creates a fire risk.

Don't delay if someone is at risk. Heat stroke can be fatal. If anyone in your household is showing signs of confusion, hot dry skin, rapid pulse, or loss of consciousness, call 911 immediately — that's a medical emergency, not an HVAC problem.

Information reflects current HVAC best practices and CDC heat safety guidelines as of early 2026, with cost data sourced from HomeGuide, Angi, and HomeAdvisor. For city-specific HVAC pricing, see our HVAC repair cost guide for Houston. For water emergencies, see what to do when a pipe bursts.