How a gas furnace starts up
A modern gas furnace runs through the same steps every time the thermostat calls for heat. The control board starts the inducer motor, a small fan that pulls air through the heat exchanger and pushes exhaust out the flue. A pressure switch confirms the inducer is moving air. The hot surface igniter glows, the gas valve opens, and the burners light. A flame sensor proves there is flame within a few seconds, or the board shuts the gas off. Once the heat exchanger warms up, the blower starts and pushes warm air through the ducts.
Throughout the cycle a limit switch watches the temperature inside the furnace and shuts the burners down if it gets too hot, usually because airflow is choked. If any step fails, the board stops, waits and tries again, and after a few failed tries it locks out. Most boards flash a code on an LED that tells the tech which step failed.
What is usually wrong
| What you notice | Likely cause | Usually |
|---|---|---|
| Burners light, then shut off after a few seconds | Dirty or failed flame sensor | Cleaning or small part |
| Inducer runs, nothing lights | Cracked igniter or a pressure switch that will not close | Part |
| Nothing happens at all | Thermostat, tripped breaker, blown board fuse or open door switch | Check, then part |
| Burners cycle on and off while the blower runs | Limit switch tripping from a clogged filter or blocked returns | Filter, airflow fix |
| Blower runs, air from the vents is cold | Burners locked out, or thermostat fan set to ON | Diagnosis |
| Loud squeal or grinding from the cabinet | Blower motor bearings or inducer motor wearing out | Motor replacement |
| Error code flashing on the board | Logged fault for a specific step | Read the code chart |
| Water around a high-efficiency furnace | Clogged condensate trap or drain line | Cleaning |
The single most common cause of furnace trouble is still a dirty air filter. A clogged filter starves the blower, the heat exchanger overheats, the limit switch trips and the furnace short-cycles until something wears out. Check the filter before you call; it costs a few dollars and sometimes solves the problem outright.
What an HVAC technician checks
- The error code on the board, before power is cycled, because a reset can clear the history.
- The thermostat and power: batteries, settings, the breaker and the service switch near the furnace.
- The startup sequence, watched step by step: inducer, pressure switch, igniter, flame, blower.
- Flame sensor current with a meter, and igniter resistance, rather than replacing parts on a guess.
- Airflow: the filter, the blower wheel, the return grilles and static pressure in the ducts.
- Venting and combustion: flue connections, the condensate drain on high-efficiency units, and carbon monoxide in the flue gas and around the furnace.
- The heat exchanger, visually and sometimes with a camera, if anything points to a crack.
Typical furnace repair prices
| Repair | Typical range | What moves the price |
|---|---|---|
| Diagnostic visit | $90–$200 | After-hours timing and travel |
| Flame sensor clean or replace | $100–$250 | Cleaning vs a new sensor |
| Hot surface igniter | $150–$350 | Silicon carbide vs nitride, brand |
| Pressure switch | $150–$400 | Single vs two-stage furnace |
| Limit switch | $150–$350 | Plus any airflow fix it reveals |
| Inducer motor | $350–$800 | Brand and whether the housing comes with it |
| Blower motor | $400–$1,200 | PSC vs variable-speed ECM motor |
| Control board | $300–$800 | Universal vs brand-specific board |
Two parts are cheap and fail often: the flame sensor and the igniter. Two are expensive and fail less often: the variable-speed blower motor and the control board. If a quote lands at the high end, ask which test pointed to that part. A competent tech will show you the meter reading or the code.
Repair or replace the furnace
A gas furnace commonly lasts 15 to 20 years with yearly service. Past about 15 years, start weighing the repair against the cost of a new unit, especially if the part is a motor or a board.
Replacement makes more sense when the heat exchanger is cracked, when the furnace is an old standing-pilot model in the 60s or 70s for AFUE, when repairs keep coming every season, or when rooms have always been uncomfortable because the unit was sized poorly. New furnaces are sold as about 80 percent AFUE for chimney-vented installs or 90 percent and above for condensing models that vent through plastic pipe. If the cooling equipment is also old, pricing a matched system or a heat pump at the same time can make sense.
Safety: gas, carbon monoxide and the heat exchanger
Smell gas? Leave the house right away without operating switches or appliances, and call your gas utility or 911 once you are outside.
A cracked heat exchanger or a disconnected flue can let carbon monoxide into the air the blower moves through the house. A CO alarm sounding, flu-like symptoms that ease outdoors, soot near the burners, or a burner flame that rolls out of the burner area are reasons to switch the furnace off at its service switch, open windows if it is safe, and get everyone outside. Do not bypass a limit or rollout switch to keep a furnace running; those switches exist to stop fires and CO problems.
Seasonal maintenance
- Change or check the filter every one to three months in heating season, more often with pets or renovation dust.
- Book a tune-up in early fall, before the first cold snap fills every HVAC company's schedule.
- Keep returns and supply registers open. Closing many vents raises static pressure and can trip the limit.
- Clear the area around the furnace and keep paint, gasoline and cleaning chemicals away from it.
- On high-efficiency furnaces, keep intake and exhaust pipes outside free of snow, ice and nests.
- Test CO alarms on every level and replace them on the schedule printed on the alarm.
What to ask the HVAC contractor
- Are you licensed for HVAC and gas work in my area, and insured?
- Do you service my furnace brand, such as Carrier, Trane, Lennox, Goodman, Rheem or York, and stock its common parts?
- What is the diagnostic fee, and does it count toward the repair?
- Which test pointed to the part you want to replace?
- What warranty comes with the part and the labor?
If your home heats with hot water radiators instead of ducts, see boiler repair. For heat pumps, thermostats and uneven heat in general, see heating repair.
Questions people ask
How much does furnace repair cost?
Most repairs run $150 to $650 including the visit. Flame sensors and igniters sit at the low end; blower motors and control boards on variable-speed furnaces can pass $1,000.
Why is my furnace blowing cold air?
Often the burners have locked out after a failed ignition or a tripped limit, while the blower keeps running. Check that the thermostat fan is set to AUTO, change the filter, and note any blinking code for the technician.
Can I clean a flame sensor myself?
Some homeowners do, but it means removing a part from the burner area with power and gas off. If you are not comfortable working inside the furnace, a tech can clean and test it quickly.
What does the blinking light on my furnace mean?
It is a diagnostic code. The chart is usually printed on the inside of the furnace door. Write down the pattern before turning power off, because cycling power can clear it.
Is a cracked heat exchanger dangerous?
It can be, because it may let combustion gases, including carbon monoxide, mix with house air. Most techs will shut the furnace down and recommend replacing the heat exchanger or the furnace.
How often should a furnace be serviced?
Once a year, ideally in early fall. A tune-up catches weak igniters, dirty sensors and airflow problems before the coldest nights.