Furnace Not Turning On: Troubleshooting and Repair Costs
Before calling for emergency service, work through these troubleshooting steps in order. Many furnace no-heat situations can be resolved without a service call, saving you $150 to $300 in diagnostic and repair fees.
Check the Thermostat Settings
This is the most common cause of "furnace not working" calls, and it costs nothing to fix. Verify that the thermostat is set to "Heat" mode rather than "Cool" or "Off." Set the target temperature at least 3 to 5 degrees above the current room temperature to trigger a heating call. If your thermostat uses batteries, replace them, as dead batteries can prevent the thermostat from communicating with the furnace. Programmable and smart thermostats occasionally lose their settings after a power outage, so check the schedule to confirm it has not reverted to a default that keeps the temperature low during current hours.
If the thermostat display is completely blank, the thermostat has no power. Check the batteries, check the circuit breaker that powers the furnace (which also powers most thermostats through the low-voltage transformer), and inspect the wiring connections behind the thermostat faceplate.
Check the Power Supply
Gas furnaces require electricity to operate the control board, blower motor, ignitor, and safety controls. Look for the power switch on or near the furnace itself. This switch looks like a standard light switch and is sometimes accidentally turned off by someone who does not realize it controls the furnace. Verify that it is in the "On" position.
Next, check the circuit breaker panel for the furnace circuit. If the breaker has tripped, reset it by flipping it fully to "Off" and then back to "On." If the breaker trips again immediately or shortly after the furnace tries to start, there is an electrical problem within the furnace (often a shorted blower motor) that requires professional repair. Do not repeatedly reset a tripping breaker.
Inspect the Air Filter
A severely clogged furnace filter restricts airflow to the point where the heat exchanger overheats, triggering the high-limit safety switch that shuts the furnace down. Remove the filter and inspect it. If it is visibly packed with dust and debris, replace it with a new filter of the same size and type. After installing a clean filter, reset the furnace by cycling the power switch off for 30 seconds and then back on.
If the furnace starts after a filter change, the clogged filter was the problem. Set a reminder to change the filter every 1 to 3 months to prevent this from happening again. A clogged filter does more than stop the furnace temporarily: running a furnace with restricted airflow stresses the heat exchanger and can shorten its lifespan.
Check the Gas Supply
The gas valve near the furnace must be in the open position for the furnace to receive fuel. The valve handle should be parallel to the gas pipe (open). If it is perpendicular (crosswise) to the pipe, the valve is closed. Turn it to the open position if it was closed.
Check whether other gas appliances in the home are working, such as the stove, water heater, or gas fireplace. If none of them work, the gas supply to the home may be interrupted. Contact your gas utility to check for outages or service interruptions in your area. If other gas appliances work but the furnace does not, the problem is with the furnace rather than the gas supply.
Reset the Furnace
Modern furnaces have a lockout feature that shuts the system down after three to five failed ignition attempts to prevent gas from accumulating. This lockout is a safety feature, not a malfunction. To reset it, turn the furnace power switch off, wait at least 30 seconds, and turn it back on. The furnace will attempt its startup sequence again.
If the furnace locks out again after the reset, the underlying cause of the ignition failure needs to be addressed. Common causes include a failed ignitor, a dirty flame sensor, a faulty gas valve, or a control board issue. At this point, professional diagnosis is recommended.
Check the Condensate Drain
High-efficiency condensing furnaces (90%+ AFUE) produce water as a byproduct of the condensing process. This water drains through a small PVC pipe, usually routed to a floor drain or condensate pump. If the drain line clogs with sediment, algae, or debris, water backs up and triggers a safety switch that prevents the furnace from operating.
Locate the condensate drain line and check for blockage. You can clear a clogged line by disconnecting it and flushing it with warm water or a mixture of warm water and vinegar. If the drain runs to a condensate pump, check that the pump is working and the float switch is not stuck in the raised position.
Call a Professional
If you have checked all of the items above and the furnace still will not start, the problem involves an internal component that requires professional diagnosis and repair. Note the error code flashing on the furnace control board LED before you call, as this information helps the technician prepare the right parts and reduces diagnostic time.
Common Repair Costs When Professional Service Is Needed
If the self-troubleshooting steps above do not resolve the problem, here are the most common professional repairs and their costs.
Ignitor replacement: $150 to $400. This is the single most common cause of a furnace that will not start after the basic checks are cleared. The ignitor is a wear item that lasts 3 to 7 years. See our ignitor replacement cost guide for more details.
Flame sensor cleaning or replacement: $80 to $300. A dirty flame sensor cannot detect the burner flame, causing the furnace to shut down within seconds of igniting. Cleaning costs $80 to $200, while replacement costs $150 to $300. See furnace pilot light and flame sensor problems.
Control board replacement: $400 to $800. The control board is the furnace's brain, sequencing each step of the startup and operating cycle. A failed board prevents the furnace from starting its ignition sequence at all. Board replacement is a moderately expensive repair but is worthwhile on furnaces under 12 years old.
Gas valve replacement: $300 to $700. The gas valve controls fuel flow to the burner. If the valve fails to open, no gas reaches the burner and the ignitor heats but nothing ignites. Gas valve replacement involves disconnecting gas lines and requires a licensed technician.
Draft inducer motor replacement: $200 to $600. The draft inducer motor creates the airflow needed to safely exhaust combustion gases. If it fails, the furnace safety controls prevent the ignition sequence from starting because exhaust cannot be properly vented.
Blower motor replacement: $300 to $1,800. If the blower motor has failed, the furnace may start and produce heat briefly but then shut down on the high-limit switch because the heat cannot be distributed. See our blower motor replacement cost guide.
Start with the simple checks: thermostat settings, power supply, filter condition, and gas supply. These free fixes resolve many no-heat situations. If the furnace still will not start, note the error code on the control board and call a licensed HVAC technician. Professional repairs for common ignition failures cost $150 to $600 in most cases.