Carbon monoxide alarm: get everyone out, then call 911
Carbon monoxide has no smell and can kill in your sleep. The CPSC's instruction is to never ignore the alarm, get outside immediately and call 911.
What is probably happening
- A fuel-burning appliance is putting carbon monoxide into the house: furnace, water heater, gas stove, a generator, or a car in the garage.
- A cracked or corroded furnace heat exchanger is one common cause. So is a blocked flue or a generator run too close to the house.
- The CDC lists the early symptoms as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. If anyone has these, tell the 911 dispatcher.
Do this in the next five minutes
- Get everyone outside into fresh air. Do not stop to open windows or find the source.
- Call 911 from outside. Say a carbon monoxide alarm is sounding and whether anyone feels unwell.
- Anyone with a headache, nausea or confusion needs medical attention now, not later.
- Stay out until the fire department has measured the air and told you it is safe.
- Once cleared, keep the furnace and any gas appliance off until an HVAC technician has inspected and tested it.
Do not
- Do not assume it is a false alarm and reset it. The alarm cannot be argued with, and low levels still hurt you over hours.
- Do not re-enter to ventilate or to grab belongings.
- Do not run the furnace again until it has been tested, even if the alarm stops once the house airs out.
Who to call
911 first, then a licensed HVAC technician
The fire department finds and measures the gas. An HVAC technician finds and fixes the appliance that made it, and tests the heat exchanger and flue before it runs again.
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What it should cost
$100 to $1,500
Furnace heat exchanger repair, national range. Furnace repairs overall average $318
Source: Angi, furnace repair cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Will you test with a combustion analyzer and show me the CO reading before and after?
- If you say the heat exchanger is cracked, can you show me the crack on camera or in person?
- Is my flue and venting checked as part of this visit?
- Do you carry EPA Section 608 certification and a state HVAC license I can look up?
End the call if
- A replacement quote for the whole system with no combustion readings shown to you. The California CSLB documented a technician who claimed a cracked, CO-leaking heat exchanger on a unit that an independent inspection found working properly.
- Anyone telling you to go back inside before the fire department has cleared the house.