Furnace Not Igniting

Most common ignition failure on modern furnaces. The HSI cracks or burns out after 5-10 years of use. Without ignitor heat, the gas valve opens but no flame.

Power on and call for heat. Within 15-30 seconds the ignitor should glow bright orange-red. If you see no glow but hear the gas valve open (and possibly smell unburned gas), the ignitor has failed.

The pressure switch verifies the inducer fan is creating proper draft before allowing ignition. If the switch is bad, or sees inadequate pressure, ignition never starts. Common failure on high-efficiency furnaces.

LED blink code typically shows pressure switch fault (varies by manufacturer). Inducer fan runs, but ignitor never glows. Could also be a clogged inducer vent pipe.

The electronic gas valve has failed and won't open even when commanded. You'll see ignitor glow but no flame. No gas smell during the ignition attempt.

Multimeter test — 24V should appear across the gas valve terminals at the moment ignition is called. If voltage is present but valve doesn't open, the valve has failed mechanically.

The inducer (draft fan) creates negative pressure to draw combustion gases through the heat exchanger. If it doesn't run or runs weakly, the pressure switch never closes and ignition is blocked.

Listen at startup for the inducer fan. Should hear it spin up within 5-10 seconds. No sound = failed motor or wiring. Weak sound = motor failing.

After 3 failed ignition attempts (because the dirty flame sensor doesn't detect flame), the control board locks out. The furnace won't try again until reset.

If the furnace ignited briefly then shut off, then no attempts after, lockout is likely. Power-cycle the furnace (off 30 seconds, on) to clear lockout.

The furnace's circuit board has failed. May show no LED, intermittent operation, or random fault codes that don't match symptoms.

Visual inspection: burn marks on the board, swollen capacitors, loose connections. Erratic blink codes that don't match documented faults. Common after lightning strikes or power surges.

Manual gas shutoff valve closed at the furnace, or gas pressure too low to support ignition.

Find the manual valve on the gas line at the furnace. Handle parallel to pipe = open. Other gas appliances working confirms house gas is fine — issue is local to the furnace.

After multiple failed ignition attempts, the control board enters lockout mode for safety. Won't try ignition again until reset.

If furnace had been trying then went silent with no further attempts, lockout is likely. Some boards show a specific lockout code.