A gas smell is not a wait-and-see problem
If you smell gas near the furnace or anywhere in the home, do not try to find the leak yourself. Do not flip switches or use the phone inside the building.
Leave the building, get to a safe location, and call your gas utility or emergency services. Once the gas safety issue is handled, call a qualified HVAC technician for the furnace.
A yellow or flickering flame deserves attention
A steady blue flame is typical for many gas furnaces. A yellow or flickering flame can indicate incomplete combustion, which may be connected to a venting, airflow, or heat exchanger issue.
Turn the system off and arrange an inspection. Do not ignore a flame color change because the house still feels warm.
Soot, corrosion, or a strange smell
Soot around the furnace, unusual odors, or visible corrosion can point to a combustion or venting problem. These are not cosmetic issues.
A cracked heat exchanger is a serious safety concern because it can allow carbon monoxide into the living space. Only a qualified technician can inspect it properly.
Banging, squealing, or rumbling
A single mild sound at startup may not be serious. Repeated banging, squealing, grinding, or rumbling usually points to a mechanical issue that should be checked.
Turn the system off if the sound is severe or accompanied by a burning smell, smoke, or repeated breaker trips.
The furnace runs but the house stays cold
A dirty filter, restricted airflow, thermostat problem, or failing component can prevent the system from delivering heat even when it runs.
Check the thermostat and filter first. If the system still will not keep up, schedule service before the next cold snap.
What to tell the technician
Share when the symptom started, what you smelled or heard, whether the system has been serviced recently, and what you have already tried.
If you have a carbon monoxide alarm, mention whether it has sounded. If it has, follow the alarm instructions and treat it as an emergency.