Furnace Not Turning On? 7 Things to Check Before You Call for Repair
Furnace not turning on? In most cases it is a thermostat, power, or filter problem, not a dead furnace. Work through these seven checks in order. A furnace that will not turn on is most often a power, thermostat, or filter problem, all checkable in under 10 minutes without tools. If you get through all seven and the furnace still will not start, call Air Supply Heating and Air Conditioning at (661) 947-9150 for same-day service in Lancaster and the Antelope Valley.
The 7 Checks
Do these in order. Stop when the furnace starts.
1. Thermostat mode and temperature. Set the thermostat to HEAT and raise the setpoint several degrees above room temperature.
2. Thermostat batteries. Replace them, even if the display looks fine. Dead batteries are one of the most common causes of a furnace that will not start.
3. Breaker. Check the furnace breaker in your electrical panel. Flip it fully off, then fully on.
4. Furnace power switch. Look for a wall switch near the furnace that looks like a light switch. Make sure it is on.
5. Air filter. A clogged filter can trip the high-limit safety switch and shut the furnace down. Replace the filter if you cannot see light through it.
6. Gas valve. The gas valve handle should run parallel to the pipe, which means open. If you smell gas, leave the house and call the gas company from outside.
7. Reset button, once. On high-efficiency furnaces, check the condensate drain for clogs and standing water. Then press the reset button, usually a red or yellow button on the blower housing, exactly once. Never press it more than once: a repeatedly tripping furnace is signaling a fault that needs a technician.
Why Furnaces Fail at First Startup in the Antelope Valley
In the Antelope Valley, furnaces often sit idle for 7+ months, so the first cold snap is when dead thermostat batteries and dusty flame sensors reveal themselves. Dust coats the flame sensor through the long cooling season, so the control board shuts the gas valve and the furnace will not start. Run our fall furnace maintenance checklist in September or October to catch these problems before the rush.
When to Stop and Call a Professional
Stop and call a technician if you smell gas, if the furnace trips again after one reset, if water is pooling around the unit, or if you notice burning or plastic smells. If the furnace runs but the air stays cold, read our guide on why a heater blows cold air. Our Furnace Repair service page covers what a diagnostic visit includes, and our contact page gets you scheduled the same day.
Frequently Asked Questions
Why won't my furnace turn on at all?
No power, a thermostat issue, or a tripped safety switch are the top causes. Confirm the thermostat is set to HEAT with fresh batteries, check the furnace breaker and the wall switch near the unit, and look at the air filter. A clogged filter can trip the high-limit switch and shut the system down. If all three check out, the problem is likely the igniter, flame sensor, or control board. Call Air Supply at (661) 947-9150 for same-day furnace repair.
Where is the furnace reset button?
Usually a red or yellow button on the blower housing or near the burner assembly inside the furnace cabinet. Press it once and wait for the furnace to cycle. Never press the furnace reset button more than once. A repeatedly tripping furnace is signaling a fault that needs a technician, such as a failed igniter or a safety switch doing its job. Repeated resets can let a dangerous condition continue. If it trips again after one reset, call a professional.
Can a thermostat keep a furnace from turning on?
Yes. Thermostat problems are among the most common reasons a furnace will not start. Dead batteries are the top offender, especially in the Antelope Valley where the thermostat sits untouched for months. The thermostat may also be set to COOL or OFF instead of HEAT, or the setpoint may be below room temperature so the furnace never gets a call for heat. Before assuming the furnace is broken, try fresh batteries, HEAT mode, and a higher setpoint.
Should I turn my furnace on before winter in Lancaster?
Yes. Test it on a mild fall day, well before the first freezing night. A fall test-run finds problems on your schedule instead of during the first-cold-snap repair rush. Let the furnace run a full cycle, check that warm air comes from the vents, and listen for unusual noises.
How much does it cost to fix a furnace that won't turn on?
Many causes are free DIY fixes: fresh thermostat batteries, a flipped breaker, a clean filter, or an opened gas valve. For professional repairs, ignition issues typically range $150 to $400 per national 2026 cost data from sources like Angi and HomeAdvisor. Exact pricing requires an on-site estimate from Air Supply Heating and Air Conditioning. Call (661) 947-9150 for same-day service.
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