If your furnace clicks, hums, and runs its startup sequence but never lights, and the house is getting cold, the odds are strong that a $30 part is the entire problem. A furnace ignitor replacement is one of the most common and most DIY-friendly furnace repairs there is, because the hot surface ignitor is a wear item that fails on a schedule, usually every three to seven years. I have swapped hundreds of them, and the part costs a fraction of what an emergency service call runs. Knowing how to diagnose and replace it can save you a $200 visit and a cold night.
This guide covers how the ignitor works, the symptoms of a bad one, how to confirm it with a meter, the replacement steps, and what it costs whether you do it yourself or call a technician.
How a Hot Surface Ignitor Works
Most furnaces built in the last few decades use a hot surface ignitor, an HSI, instead of a standing pilot light. It is a small, fragile element, historically silicon carbide and more recently silicon nitride, that glows orange-hot when current passes through it. When the thermostat calls for heat, the control board energizes the ignitor, it heats to well over 1,800°F, the gas valve opens, and the gas ignites off the glowing element. A flame sensor then confirms the burner lit, and the ignitor de-energizes.
Because it heats and cools through that extreme temperature swing every single cycle, the element gets brittle and eventually cracks or burns open. That thermal fatigue is why the HSI is a maintenance item rather than a lifetime part. When it opens, no current flows, no glow, no ignition.
Symptoms of a Failed Ignitor
A dead ignitor produces a recognizable pattern:
- The furnace runs through its sequence, you hear the inducer fan and the click of the gas valve, but the burners never light.
- No orange glow visible through the burner sight window when the furnace tries to start.
- The furnace attempts ignition several times, then locks out, often flashing an error code on the control board.
- A visible crack or a blistered white spot on the ignitor element.
Read the LED error code flashing on the control board through the sight window; most furnaces flash a specific code for ignition failure. That, combined with no glow, points squarely at the ignitor, though a bad flame sensor, a closed gas valve, or a control board fault can produce similar no-heat symptoms.
Confirm the Ignitor With a Meter
You can test an HSI without even removing it in many cases. Kill power at the furnace switch and the breaker first. Set a multimeter to measure resistance in ohms, then measure across the two ignitor leads after disconnecting them from the harness.
A healthy silicon carbide ignitor typically reads somewhere in the range of 40 to 90 ohms, and silicon nitride units read differently, so check the spec for your specific part. The critical result is this: if the meter reads infinite resistance, an open circuit, the element is cracked internally and the ignitor is dead. If it reads a normal resistance value and the furnace still will not light, the problem is elsewhere, in the gas valve, the flame sensor, or the control board, and swapping the ignitor will not fix it. That five-minute test saves you from throwing parts at the wrong problem.
Handle the New Ignitor Carefully
Hot surface ignitors are shockingly fragile, and the number one cause of premature failure is a person touching the element. The silicon carbide type is as delicate as a light-bulb filament, and oils from your skin create hot spots that shorten its life. When you install the new one, hold it only by the ceramic base and the wiring, never by the glowing element itself. If you touch the element, wipe it with rubbing alcohol before installing.
Match the replacement to your furnace. Universal HSI kits exist, but the correct approach is to match the part number, the mounting bracket, the connector type, and the element geometry to your specific furnace model. A mismatched ignitor may sit at the wrong angle relative to the burner and fail to light the gas reliably.
Step-by-Step: Replacing the Ignitor
With power off and the correct part in hand, the swap is quick:
- Turn off power at the furnace switch and breaker, and shut the gas valve if you want the extra margin of safety.
- Remove the burner access panel.
- Locate the ignitor at the end of the burner assembly, held by one or two screws, with a two-wire connector.
- Unplug the wire connector and remove the mounting screw.
- Carefully lift out the old ignitor, noting its exact position and angle relative to the burner.
- Install the new ignitor in the same position, holding it only by the base, secure the screw, and plug in the connector.
- Replace the panel, restore power and gas, and run a full heating cycle to confirm the burners light and stay lit.
Watch the first ignition through the sight window. You should see the element glow bright orange, the gas ignite, and a steady blue burner flame. If it lights and holds, you are done.
Silicon Carbide vs Silicon Nitride Ignitors
Not all hot surface ignitors are the same, and knowing which type you have affects how long it lasts and what to buy. The older technology is silicon carbide, a flat or spade-shaped element that is extremely fragile and typically lasts three to five years. It is the type most prone to cracking from thermal cycling and from being touched during installation. If your furnace is older, this is almost certainly what it uses.
The newer technology is silicon nitride, a more robust element, often rod-shaped, that resists thermal shock far better and commonly lasts seven years or more. Many universal replacement kits use silicon nitride even for furnaces that shipped with silicon carbide, and upgrading to one can extend the interval between failures. The two types have different resistance values and sometimes different voltage requirements, so if you switch types, use a properly matched universal kit designed for your furnace rather than assuming any ignitor will drop in. When in doubt, match the OEM part number for a guaranteed fit, or buy a reputable universal kit that lists your furnace model.
Why Ignitors Keep Failing
If you find yourself replacing the ignitor every year or two, something beyond normal wear is at work, and swapping the part again without addressing the cause just repeats the cycle. A few underlying problems cause repeat failures.
- Touching the element during install leaves skin oils that create hot spots and crack the ignitor early. Always handle by the ceramic base.
- Poor grounding at the furnace can cause the ignitor and flame-sensing circuit to misbehave and overstress the element.
- Voltage problems from a failing control board can overdrive the ignitor and burn it out fast.
- A cracked heat exchanger or a flame-rollout condition can disturb the burner flame and stress components.
- Short-cycling from an oversized furnace or a bad limit switch multiplies the thermal cycles the ignitor endures, aging it faster.
If a new ignitor fails within a season, stop replacing it blindly and have a technician check the grounding, the control board voltage, and the burner condition. The repeat failure is a symptom of a deeper fault.
Costs and When to Call a Pro
The ignitor itself is cheap. A hot surface ignitor part runs $15 to $60 depending on the model and whether you buy OEM or a quality aftermarket unit. That makes DIY replacement one of the best-value furnace repairs going, often turning a $200 to $350 service call into a 20-minute job and a $30 part.
Call a pro if your furnace is under warranty, if you measured a good resistance value and it still will not light, meaning the fault is elsewhere, or if you are not comfortable working around the gas burner assembly. Also step back if the ignitor keeps failing quickly, because repeated failures point to a deeper issue such as poor grounding, a cracked heat exchanger causing flame disturbance, or a control board sending the wrong voltage. For a straightforward cracked-ignitor no-heat call, though, this is about as approachable as furnace work gets, and it is worth keeping a spare ignitor on the shelf if you own an older furnace.
