What should you do first?
Set the thermostat to Heat and fan to Auto, raise the setpoint, and check the filter. If a CO alarm sounds or gas is suspected, leave immediately and call 911 or the gas authority from outside. Do not keep cycling a furnace that repeatedly fails ignition.
Safe first checks
Before requesting repair
- Heat mode, fan Auto, and setpoint above room temperature
- Correct filter and open vents
- Furnace switch and breaker checked once if safe
- Diagnostic light pattern noted without opening panels
Stop and call
Warning signs that need more caution
- CO alarm, gas odor, dizziness, nausea, or confusion
- Soot, flame rollout, smoke, or a booming ignition sound
- Repeated start-and-stop cycle without heat
- Breaker trips or electrical burning odor
- Indoor temperature falling toward an unsafe level
Possible causes
What may be behind the symptom
Thermostat or fan setting
Fan On can circulate unheated air between cycles, while a thermostat or wiring fault can interrupt a heat call.
Airflow safety shutdown
A blocked filter, blower problem, or restricted duct can overheat the furnace and open a limit switch.
Ignition, fuel, or venting fault
Igniters, flame sensors, gas valves, pressure switches, and venting require professional service.
Professional diagnosis
What a technician may inspect
- Thermostat demand and fault codes
- Ignition sequence and flame proving
- Gas controls, venting, and safety devices
- Filter, blower, temperature rise, and limit operation
- Electrical supply and control board
Cost context
What can affect repair price?
- The system type, capacity, age, and accessibility
- The failed component and whether compatible parts are available
- Diagnostic or service-call fees in the local market
- After-hours timing and current provider availability
- Refrigerant type, electrical work, and any additional testing required
Common questions
Frequently asked questions
Why does the furnace blow cold air?
The fan may be set to On, the burners may not be lighting, a limit may have shut heat down, or the system may be between cycles. Persistent cold air needs diagnosis.
Can I clean a flame sensor myself?
Avoid opening combustion equipment or bypassing safeties. A technician should confirm the fault rather than replacing or cleaning parts by guesswork.
What if the CO alarm sounds?
Get everyone outside immediately, then call 911. Do not re-enter until responders authorize it.
How do I prevent pipes from freezing while waiting?
Use safe, code-compliant temporary measures and protect vulnerable plumbing, but never use an oven, charcoal grill, or indoor generator for heat.