Homeowner Safety Guide
If a carbon monoxide alarm sounds or you suspect CO poisoning, get everyone outside to fresh air immediately and call 911. Do not search for the source, reset the alarm to keep using the furnace, or wait for symptoms.
Leave first. Call from a safe place outside. Do not reenter until emergency responders say it is safe.
Call 911 from a safe locationCPSC carbon monoxide safety guidanceOnce outside, call 911 and check that everyone is accounted for. Tell responders if anyone has symptoms. Do not go back inside until emergency responders give permission, even if the alarm stops. Do not open furnace panels or cycle the thermostat to investigate.
The useful details are the alarm location, whether anyone felt symptoms, whether the furnace had been running, and whether there was a gas smell, smoke, soot, blocked vent concern, or recent work. The first job is making the home safe, not proving the furnace is the cause.
Read the gas smell safety guideAfter emergency responders, the gas utility, or the appropriate safety path clears the immediate CO concern, a heating diagnostic can look at the furnace sequence, venting clues, burner behavior, airflow, and why the system should or should not run. Normal no-heat service can start with the $95 Springfield Diagnostic. Nearby towns may cost more.
Read the furnace no-heat guide