Homeowner Safety Guide
A carbon monoxide alarm or CO detector concern around a furnace is not a normal no-heat troubleshooting step. Get people and pets to fresh air, call emergency services or the appropriate local safety path from a safe location, and do not keep running the furnace to see if the alarm comes back.
If a CO alarm is sounding, leave the home and contact emergency services or the appropriate local safety authority from fresh air. Do not ignore the alarm because the furnace still seems to be heating, and do not open panels or cycle the thermostat to create another clue.
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