Furnace Won’t Ignite: Causes & Fixes

1. Clogged or Dirty Air Filter (Start Here)

A severely restricted filter can cause the furnace to overheat and trip its high-limit safety switch, preventing ignition entirely. This is the easiest and most overlooked fix — always check it first.

1. Turn the thermostat off and switch the furnace power switch (usually on the unit itself) to OFF.
2. Locate the filter slot, typically at the return air duct or the bottom of the furnace cabinet.
3. Slide out the filter and hold it up to a light. If you cannot see light through it, it needs replacing.
4. Insert a new filter with the airflow arrow pointing toward the blower motor.
5. Restore power and set the thermostat to HEAT, 5°F above room temperature. Wait 2–3 minutes for the ignition cycle to start.

2. Dirty Flame Sensor

The flame sensor is a thin metal rod that confirms a flame is present; when it’s coated with oxidation or residue, it can’t do its job and the furnace shuts off the gas as a safety measure — one of the top reasons a furnace won’t ignite and stay lit.

1. Turn off the furnace power switch and shut off the gas supply valve to the furnace.
2. Remove the furnace cabinet door (usually two screws or a lift-and-pull release).
3. Locate the flame sensor — a single metallic rod mounted on a bracket near the burners, connected by one wire.
4. Remove the one mounting screw and gently pull the sensor out.
5. Lightly rub the metal rod with fine-grit steel wool or a clean dollar bill (abrasive enough to remove oxidation) until it is shiny. Do not use sandpaper — it removes too much material.
6. Reinstall the sensor, reconnect the wire, and reassemble the cabinet.
7. Restore gas and power. Run a heat cycle and observe: the burners should ignite and stay on.
8. If cleaning doesn’t help, replace the flame sensor.

3. Failed Hot Surface Ignitor

The hot surface ignitor (HSI) glows orange-hot to light the gas burners; if it is cracked or burned out, the gas will never light. This is the single most common part failure when a furnace won’t ignite.

1. Turn off the power switch and gas supply to the furnace.
2. Remove the access panel and locate the ignitor — it sits in the burner assembly and looks like a small, fragile silicon carbide or silicon nitride fork or flat blade.
3. Inspect it visually: a crack, chip, or broken element means it must be replaced. Even hairline cracks cause failure.
4. If you have a multimeter, set it to resistance (Ohms) and test across the ignitor terminals. A functional silicon carbide ignitor typically reads 40–100 Ω; an open reading (OL/infinite) means it is failed.
5. Disconnect the plug connector and remove the single mounting screw. Do not touch the ignitor element with bare hands — skin oils cause premature failure. Use gloves or a clean cloth.
6. Install the new ignitor, reconnect the plug, and replace the access panel.
7. Restore gas and power. Watch through the sight glass (if present) for the ignitor to glow, then for the burner to light within 5–10 seconds.

4. Tripped Breaker or Blown Furnace Fuse

Furnaces require a steady 120V power supply, and many have an internal 3A or 5A fuse on the control board that protects the system — a blown fuse means zero ignition activity of any kind.

1. Go to your home’s electrical panel and check for a tripped breaker labeled “Furnace” or “HVAC.” Reset it firmly to OFF, then back to ON.
2. If the breaker looks fine, turn off the furnace power switch and open the cabinet.
3. Locate the control board. Look for a small blade-style automotive fuse (usually 3A, 5A, or 7.5A) in a dedicated fuse holder on the board.
4. Pull the fuse and inspect it — a broken filament or dark interior means it has blown.
5. Replace with an identical amperage fuse. Never use a higher-amp fuse — this can damage the control board.
6. If the new fuse blows immediately, stop and call a technician; there is likely a wiring short.

5. Faulty Pressure Switch

The pressure switch confirms that the inducer motor is creating the proper draft before allowing ignition; if the switch fails or its rubber hose is cracked, the furnace will lock out and refuse to ignite.

1. With power off, open the cabinet and locate the pressure switch — a round disc (1–2 inches diameter) with a rubber hose attached, connected to the inducer motor housing.
2. Inspect the rubber hose for cracks, kinks, or disconnection. A split hose is a cheap, easy fix: replace the hose with the same diameter tubing from a hardware store.
3. Check the hose and the port on the inducer for any water (condensate) blockage. Blow gently through the hose to clear it.
4. If the hose is fine, use a multimeter set to continuity. With the furnace powered up and the inducer running, the pressure switch should click closed (showing continuity). A switch that stays open while the inducer runs is defective.
5. Note the pressure rating printed on the switch (e.g., –0.65″ WC) and order a direct replacement rated the same.
6. Disconnect the wires and hose, swap the switch, reconnect, and test.

6. Failed Gas Valve or Control Board

If all the above components test good but the burners still won’t fire, the gas valve or main control board may be at fault — these are higher-cost repairs best confirmed with a multimeter and, ideally, professional diagnostics.

1. Confirm your gas supply is on: check that other gas appliances (stove, water heater) are working. If none work, contact your gas utility immediately.
2. With the furnace in a call-for-heat cycle, use a multimeter to verify 24–28V AC is reaching the gas valve terminals when it should be open. No voltage means the control board isn’t sending the signal — suspect the board.
3. If 24V is present at the gas valve but it doesn’t open, the valve itself is faulty.
4. Check the control board for visible burn marks, cracked solder joints, or a fault code displayed on the board’s LED (count the blinks and cross-reference with the label inside your furnace cabinet door).
5. Gas valve and control board replacement are within DIY reach for experienced homeowners, but given the gas safety implications, strongly consider professional service for these components.

Frequently asked questions