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The AC or heat stopped working: what to check and when to call

Safe quick checks for an AC or furnace, gas and carbon monoxide warnings, staying safe in extreme weather, hiring a tech and renters’ rights.

Quick answer

Check the thermostat mode, temperature and batteries, then the breaker, furnace switch and filter; a clogged filter can freeze AC. If you smell gas or a carbon monoxide alarm sounds, get out and call from outside. Never heat with a generator, grill or gas oven. Call a licensed HVAC tech for a written quote, and if you rent, tell your landlord in writing.

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Your checklist

  • Check the thermostat and batteries
  • Check the breaker, switch and filter
  • Gas smell or CO alarm: get out
  • Never heat with a generator, grill or oven
  • Call a licensed tech; get a written quote
  • Renting? Tell your landlord in writing

Rather read it? Here’s the whole thing.

Quick, safe checks

  • Thermostat: right mode (Cool or Heat), right temperature, new batteries.
  • Power: check for a tripped breaker, and that the furnace switch (often like a light switch near the unit) is on. The outdoor AC unit may have its own shut-off.
  • Filter: replace it if it’s dirty. A clogged filter can make AC freeze up or a furnace shut down.
  • If AC is frozen, turn it off and run the fan to thaw it.

When to stop

If you smell gas or a carbon monoxide alarm sounds, leave right away and call 911 or the gas company from outside. Don’t flip switches.

Stay safe meanwhile

Never heat your home with a generator, grill or gas oven. Keep space heaters 3 feet from anything that can burn. In extreme heat, drink water, check on older neighbors, and use a cooling center if needed.

Hiring a tech

Ask the visit fee up front and get a written quote before repairs. For big repairs or a replacement, get a second opinion.

Renting

Many places require landlords to provide working heat, and some require cooling. Report it in writing and keep a copy.

Official sources, and where to look

To find your local office, search for:

  • [your city] cooling center
  • [your state] landlord heat requirements

Pick sites that end in .gov. Official forms are almost always free.

Common questions

Why is my AC running but not cooling?

Common causes are a dirty filter, a frozen coil, a dirty outdoor unit, or low refrigerant. Check the filter and thermostat; the rest needs a tech. More ›

How long should a furnace or AC last?

Many last around 15 years or more with maintenance, but it varies. Yearly checkups help catch problems early. More ›

Why is my AC blowing warm air?

Check the thermostat is on Cool, the filter is clean, the breaker hasn’t tripped and the outdoor unit is running. If those are fine, it may be low refrigerant or a failed part; call a tech. More ›

Why is my furnace blowing cold air?

It may be the fan setting (set it to Auto), a dirty filter, or an ignition problem. If it keeps happening, call a tech. More ›

How often should I change my HVAC filter?

Often every 1 to 3 months, more with pets or allergies. Check it monthly. More ›

What are signs of carbon monoxide poisoning?

Headache, dizziness, nausea, weakness and confusion, often in several people at once. Get out and call 911. Install CO alarms near bedrooms. More ›

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Not yet reviewed by an expert. Last checked 2026-10-29. This is general help, not legal, medical or professional advice. Rules are different in every state.