Furnace Won't Turn On? Here's What to Check First
It's cold. You bump the thermostat up. Nothing happens. Before you call for emergency service — or spend a night worrying — there are a few things you can check yourself that resolve a surprising number of "furnace not turning on" situations in under ten minutes. Here's a logical order to work through them.
Start with the Thermostat
It sounds basic, but the thermostat is the starting point for a reason. Check these things first:
- Is it set to HEAT, not COOL or FAN? Thermostats get bumped more than people realize.
- Is the set temperature above the current room temperature? If the thermostat reads 68°F and you have it set to 65°F, the furnace won't turn on — it thinks it's already warm enough.
- Does it need batteries? Many digital thermostats run on AA or AAA batteries. A low-battery warning or blank screen can mean the thermostat isn't sending any signal to the furnace at all.
- Try setting it 5 degrees higher than the room temperature and waiting a minute to see if the furnace responds.
Check the Circuit Breaker
Furnaces are connected to your home's electrical system even if they run on gas — they need electricity to power the blower motor, igniter, and controls. Check your breaker panel for a tripped breaker labeled "furnace" or "HVAC."
A tripped breaker sits between ON and OFF. Switch it fully to OFF, then back to ON. If the breaker trips again immediately, there's an electrical problem that needs a professional. Don't keep resetting a breaker that keeps tripping.
Also check the power switch near the furnace itself — it looks like a regular light switch and is sometimes accidentally turned off.
Look at the Air Filter
A severely clogged air filter can actually shut your furnace down. Here's why: when airflow is blocked, the heat exchanger gets too hot and triggers the high-limit safety switch. The furnace shuts itself off to prevent damage. You might hear the ignition attempt but the system won't stay running.
Pull out the filter and hold it up to a light. If you can barely see through it, replace it with a new one of the same size. After replacing the filter, give the furnace 15–20 minutes to cool down, then try restarting it.
The EPA recommends maintaining good indoor air quality through proper filter changes — a habit that protects both your health and your HVAC system.
Check the Furnace Door Panel
Most furnaces have a safety switch inside or behind the access panel door. If that panel is slightly ajar — sometimes it shifts when someone changes a filter — the switch cuts power to the system. Make sure the door panel is firmly seated and latched.
Try the Reset Button
Many furnaces have a red or yellow reset button on the burner assembly, usually visible after opening the lower access panel. If the furnace tripped on a safety lockout, pressing this button once may restart it.
Important: only press the reset button one time. If the furnace doesn't start after one reset, stop. Pressing it repeatedly can allow unburned gas to accumulate, which is a safety risk. If one reset doesn't do it, you need a technician to find out why the lockout occurred in the first place.
If you smell gas at any point, don't touch anything. Leave the house, leave the door open, and call your gas company's emergency line from outside.
When to Call a Professional
If you've worked through the steps above and the furnace still isn't turning on, it's time for a professional. Common issues that need a tech include:
- A failed igniter (no click or glow when the furnace tries to start)
- A dirty or failed flame sensor that shuts the system down after a few seconds
- A faulty control board
- A condensate drain blockage (common in high-efficiency furnaces)
- A problem with the inducer motor or pressure switch
These aren't do-it-yourself repairs — they require diagnostic equipment and experience to troubleshoot safely.
AAA Home Services Is Ready to Help
If your furnace isn't starting and the basic checks didn't solve it, our team can diagnose and repair it. We've been serving O'Fallon and the surrounding area — including all of St. Charles, St. Louis, Lincoln and Warren Counties — since 1970. Our technicians do not work on commission, so there's no incentive to recommend parts or services you don't need.
For O'Fallon furnace repair or anywhere in our service area, call (636) 397-3200 or request service online. You can also learn more about our furnace repair services.
