Fix it
Why won't my furnace turn on?
A furnace that does nothing at all is often a chain-of-permission problem: thermostat, power, door switch, and safeties each get a veto before ignition is allowed. Walking the chain in order finds most no-starts without tools.
What it usually is
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Thermostat not calling — batteries, mode or program
Dead thermostat batteries, HEAT not selected, or a program holding a low setpoint are collectively the most common no-heat cause every winter.
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Power interrupted — switch, breaker or door panel
The furnace switch looks exactly like a light switch and gets flipped by mistake; the blower-door safety switch stops everything if the panel is not seated after a filter change.
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A tripped condensate overflow switch
High-efficiency furnaces drain condensate; a clogged line lifts a float switch that blocks ignition until the water is cleared.
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A lockout after repeated failed ignition
Controls that failed to light several times stop trying and flash an error code behind the panel sight glass. The code count is the diagnostic gift — note it before cycling power.
What you can check yourself
Set the thermostat 5 degrees above room temperature in HEAT with fresh batteries. Check the furnace switch and breaker, and reseat the blower door. Look for water around the unit and empty the condensate trap if accessible. Note any LED flash code, then cycle power once at the switch.
When it’s a job for a pro
A lockout that returns after one power cycle means the underlying igniter, sensor or pressure problem needs a meter. Bring the flash code and the model number to the call — it often turns a diagnostic visit into a repair visit.
No-heat diagnostic visits typically run $80 to $150 and are often credited against the repair; common no-start parts (igniter, flame sensor, float switch service) land between $100 and $400.
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