Introduction
A furnace that refuses to ignite can quickly turn a comfortable home into an icebox. Identifying the furnace ignition failure is the first step toward restoring heat. While some fixes are simple maintenance tasks, others involve high-voltage components. This guide ranks solutions from most likely to least likely to help you troubleshoot efficiently.
5 Common Causes for a Furnace Not Igniting
1Dirty or Oxidized Flame Sensor
Carbon buildup on the flame sensor can make the furnace light briefly, then shut down for safety.
Symptoms
- Furnace ignites for 2-5 seconds and then shuts off
- Blower runs, but no heat is produced
Care Plan
- Turn off power at the furnace switch and the circuit breaker.
- Locate the flame sensor near the burners and remove the retaining screw.
- Pull the sensor out carefully and clean only the metal rod with fine steel wool or 400-grit emery cloth.
- Wipe clean, reinstall, and restart the furnace.
- If the short-cycle symptom continues, stop and book a service pro.
Common Mistakes
- Using coarse sandpaper or chemical cleaners that damage the rod.
- Touching the porcelain base with oily hands.
2Faulty Hot Surface Igniter (HSI)
A cracked or worn hot surface igniter can prevent gas from lighting during the ignition cycle.
Symptoms
- You hear a click but never see a bright orange glow near the burners.
- Visible crack marks or a dark burn spot on the igniter element.
Care Plan
- Shut off furnace power and wait until the unit is fully cool.
- Unplug the igniter wire harness and remove the mounting screw.
- Install an exact model-match igniter and handle it only by the ceramic base.
- Restore power and run a heating cycle test.
- If the igniter still does not glow, stop and call a service pro for electrical diagnosis.
Common Mistakes
- Installing a universal igniter with the wrong resistance.
- Touching the igniter element with bare fingers.
3Blocked Pressure Switch or Inducer Port
A blocked pressure switch tube or inducer port can interrupt the safety check and stop ignition.
Symptoms
- Inducer motor runs, but igniter sequence does not continue.
- Control board flashes a pressure-switch-related error code.
Care Plan
- Turn off power and inspect the pressure-switch tubing for cracks, moisture, or loose fit.
- Remove the tube and gently clear the inducer port using a paperclip or small wire.
- Blow through the tube to confirm it is clear, then reconnect securely.
- Restart and check for a normal ignition sequence.
- If the error code returns, stop and book professional service.
Common Mistakes
- Replacing the pressure switch before checking the tube and inducer port.
- Reconnecting damp tubing without clearing condensate backup.
4Failed Gas Valve
A failed gas valve can block burner gas flow even when the igniter and controls are working.
Symptoms
- Igniter glows bright orange but burners do not light.
- No normal gas ignition sound after igniter reaches temperature.
- No gas flow signs at burners during a heat call.
Care Plan
- Verify the upstream manual gas valve is fully open and other gas appliances are working.
- Confirm the furnace is calling for heat and review any board error codes.
- Do not open, disassemble, or attempt to repair the gas valve body.
- If gas valve failure is suspected, stop and have a licensed HVAC technician test and replace it.
Common Mistakes
- Assuming the valve is bad when the manual shutoff valve is partially closed.
- Attempting DIY gas valve repair instead of licensed replacement.
5Faulty Control Board
A faulty control board can break the ignition sequence by misfiring signals to the igniter, gas valve, or safety sensors.
Symptoms
- No diagnostic LED activity on the board.
- Erratic or skipped ignition steps.
- Repeating board-related error codes.
Care Plan
- Turn off power and document all diagnostic LED codes and wiring positions.
- Check for visible board damage, moisture, corrosion, or burnt terminals.
- Verify proper incoming low voltage before replacing any board.
- If replacement is needed, install the exact model and match all switch/jumper settings.
- If behavior remains unstable, stop and schedule pro-level electrical diagnostics.
Common Mistakes
- Replacing the board before confirming transformer voltage.
- Installing a board without matching DIP/jumper settings.
