Why Won't My Furnace Ignite?
A furnace that won't ignite usually has a failed hot surface igniter, dirty flame sensor, or gas valve issue. Listen for the ignition sequence: inducer motor, then igniter glow, then burner ignition. If you hear clicking but no ignition, the igniter may be cracked. If it ignites briefly then shuts off, the flame sensor likely needs cleaning. Check for error codes on the control board.
Common Causes
Failed Hot Surface Igniter
Most modern furnaces use a hot surface igniter (HSI) that glows red-hot to ignite gas. These are fragile ceramic components that crack or wear out over time, typically lasting 3-7 years.
Dirty Flame Sensor
The flame sensor is a small rod that detects whether the burners are lit. Carbon buildup prevents it from sensing the flame, causing the gas valve to shut off as a safety measure.
Gas Valve Issues
The gas valve controls fuel flow to the burners. If it's stuck, malfunctioning, or not receiving the signal to open, the furnace can't ignite.
Pressure Switch Problems
The pressure switch verifies that the draft inducer motor is running and venting properly. If it fails or the vent is blocked, the ignition sequence stops.
Draft Inducer Motor Failure
Before ignition, the draft inducer creates airflow for safe venting. If this motor fails, the pressure switch never closes and ignition is prevented.
Control Board Failure
The control board orchestrates the ignition sequence. If it's faulty, it may not send proper signals to components even if they're all working individually.
What Should You Do?
Try This First
- Listen for the sequence: inducer motor starts, then click of igniter, then burner ignition
- Look through the sight glass to see if the igniter glows orange/red
- Check for error codes (blinking LED on the control board)
- Verify the gas valve is in the ON position
- Make sure the furnace door/panel is properly closed (safety switch)
Call a Pro If...
- You smell gas (leave immediately and call 911)
- The igniter doesn't glow when the furnace tries to start
- The furnace tries to ignite multiple times then locks out
- You see error codes flashing on the control board
- The inducer motor doesn't start at all