What to do right now
Leave the home with everyone, including pets, and call 911 or your gas utility from outside. Don't use light switches, phones or the thermostat inside. Don't go back in until emergency responders say it's safe.
After the home is cleared, an HVAC technician can find the source, which is often a cracked heat exchanger, a blocked flue or poor combustion, and repair it.
Gas smell vs carbon monoxide
| Natural gas / propane leak | Carbon monoxide | |
|---|---|---|
| Smell | Rotten eggs (added odorant) | None; CO is odorless |
| How you know | Your nose, hissing near lines | A CO alarm, or symptoms |
| Symptoms | Usually none until ignition risk | Headache, dizziness, nausea, confusion, flu-like illness |
| Call | Gas utility or 911, from outside | 911, from outside |
Common furnace and boiler CO sources
- Cracked or corroded heat exchanger
- Blocked chimney or flue (bird nests, ice, debris)
- Disconnected or sagging vent pipe
- Incorrect burner adjustment or gas pressure
- Backdrafting caused by exhaust fans or a sealed utility room
- Snow-blocked high-efficiency vent pipes
CO alarm placement
The U.S. Consumer Product Safety Commission recommends CO alarms on every level of the home and outside sleeping areas. Test them monthly and replace them per the manufacturer's date, often 5–10 years.
If an alarm chirps rather than sounds, it may be a low battery or end-of-life warning. Check the label.
What happens on the repair visit
- The technician confirms the system is safe to inspect (often after the utility has shut off the gas).
- Inspects the heat exchanger, flue, venting and burners.
- Runs a combustion analysis to measure CO in the flue.
- Checks for backdrafting with exhaust fans running.
- Repairs the fault, or red-tags the appliance if it can't be made safe.
If your furnace is red-tagged
A red tag means the appliance was found unsafe and should not be used. Don't remove the tag or relight the unit. Ask the provider about repair versus replacement, and use safe temporary heat in the meantime.
CO and gas FAQs
Can a furnace leak carbon monoxide when it's off?
Generally no; CO comes from combustion. But other appliances, such as water heaters, generators and attached-garage vehicles, can still produce it.
Does every home need a CO alarm?
Any home with fuel-burning appliances, a fireplace or an attached garage should have them. Many states require them by law.
Next steps
Ready to get it fixed? Call (888) 555-0143 to reach a licensed provider in your area. Related guides:
Busy winter markets: emergency HVAC repair in Texas · emergency HVAC repair in North Dakota · emergency HVAC repair in Minnesota · emergency HVAC repair in New York · emergency HVAC repair in Massachusetts · emergency HVAC repair in Tennessee · all states.