If your furnace turns on, runs the inducer, but never lights, a failed hot surface igniter (HSI) is one of the most common culprits. The good news: it's an inexpensive, replaceable part. This guide explains how the igniter works, how to test it, and how to swap it out safely.
How a Hot Surface Igniter Works
The HSI is a small element — usually silicon carbide or the more durable silicon nitride — that glows bright orange when current passes through it. On a call for heat, the control board energizes the igniter, it reaches well over 1,800°F, the gas valve opens, and the hot element lights the burners. A nearby flame sensor then confirms the flame is present.
Symptoms of a Failed Igniter
- No ignition. The inducer runs and you hear the gas valve click, but the burners never light.
- No glow. Watch through the inspection window — a healthy igniter glows bright orange. No glow usually means an open element.
- Visible cracks. Silicon carbide igniters are brittle and crack with age or thermal cycling. A hairline crack breaks the circuit.
- Short cycling on ignition lockout. The furnace tries several times, fails, and locks out for a cooldown period.
Testing the Igniter With a Multimeter
Turn off power first. Then disconnect the igniter's two-wire plug and measure resistance across the terminals:
- Silicon carbide igniters typically read roughly 40–90 ohms when good.
- Silicon nitride igniters read differently — check the spec for your model.
- An "OL" or infinite reading means the element is open (broken) and the igniter must be replaced.
Always handle the element by its ceramic base. Oils from your fingers create hot spots that shorten its life, so never touch the glowing portion.
Replacing a Hot Surface Igniter
- Shut off power and gas to the furnace.
- Locate the igniter next to the burner assembly. It's held by one or two screws.
- Unplug the wire harness and remove the mounting screw carefully — the element snaps easily.
- Install the new igniter in the same position and orientation so it sits in the burner flame path correctly.
- Reconnect the harness, restore gas and power, and watch a full ignition cycle through the window.
OEM Fit Matters
Igniters are not universal. Resistance value, mounting bracket, wire length, and connector all have to match. Upgrading from silicon carbide to a silicon nitride igniter can improve durability, but only when it's the correct OEM-spec part for your furnace. If your igniter keeps failing, also have the control board checked — an over-driving board can burn out igniters prematurely.
Shop genuine hot surface igniters and pair the repair with a fresh flame sensor if your furnace has also been dropping out shortly after lighting.
How Long Do Hot Surface Igniters Last?
Silicon carbide igniters typically last 3 to 5 years, while the tougher silicon nitride type often lasts longer. Every ignition cycle stresses the brittle element through thermal expansion, so furnaces that short-cycle wear igniters out faster. If you've replaced an igniter more than once in a couple of years, look for an underlying cause: an over-driving control board, low gas pressure that delays ignition, or a marginal flame sensor forcing repeated retries. Fixing the root cause protects the new igniter.
Igniter vs. Flame Sensor: Don't Confuse Them
These two parts sit near each other and are easy to mix up, but they do opposite jobs. The igniter creates the heat that lights the burners. The flame sensor only detects that a flame is present and tells the board to keep the gas valve open. If your furnace lights and then shuts off a few seconds later, the igniter is fine — the flame sensor is the more likely suspect. If it never lights at all, focus on the igniter.
Frequently Asked Questions
Can I clean a hot surface igniter instead of replacing it?
No. Unlike a flame sensor, which benefits from a light cleaning, an HSI element should never be sanded or scrubbed. Doing so removes its protective coating and weakens the brittle element. A failed igniter must be replaced.
How much does a furnace igniter cost?
Igniters are one of the more affordable furnace parts, which makes a DIY replacement very cost-effective compared with a service call. Matching the correct OEM-spec igniter for your model is what matters most for a lasting fix.
Why does my new igniter glow but the furnace still won't light?
If the element glows but burners don't ignite, the gas valve may not be opening, gas pressure may be too low, or the igniter may be positioned out of the burner flame path. Verify the valve receives voltage and that the igniter sits correctly.
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