Hot Surface Ignitor Troubleshooting — Furnace Won't Light
The hot surface ignitor (HSI) is a silicon carbide or silicon nitride element that glows to approximately 2,000°F to ignite the gas burners. It's the most commonly replaced furnace component — more often than the heat exchanger, gas valve, or control board combined. A cracked ignitor won't glow, so the gas valve never opens and the furnace locks out after several failed ignition attempts. On Carrier, Trane, Lennox, Rheem, York, and Goodman furnaces, the ignitor is accessible without tools in most cases and takes 10 minutes to replace.
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Common Symptoms
- Furnace tries to ignite but never lights
- No orange glow visible from the ignitor during startup
- Furnace locks out after 3 ignition attempts
- Error code: ignition failure, no flame detected
- Gas valve clicks open but no combustion occurs
- Furnace worked yesterday and stopped working overnight
Most Likely Causes
- 1
Cracked Hot Surface Ignitor (Most Common)
Silicon carbide ignitors are extremely fragile — a fingerprint's oil residue, a brief physical contact, or simple age-related thermal fatigue can crack them. A cracked ignitor either doesn't glow at all or glows dimly at only part of its surface. Visually inspect the ignitor element — any crack, even hairline, means it needs replacement.
- 2
Open-Circuit Ignitor (Electrical Failure)
Even without a visible crack, the internal element can fail open. Test with a multimeter in resistance mode: silicon carbide ignitors typically read 40–200 ohms; silicon nitride ignitors read 10–30 ohms. OL (open) means the element is failed. Measure at room temperature with the furnace off.
- 3
No Voltage at Ignitor
The control board supplies 120V AC to the ignitor during the ignition sequence (typically for 17–34 seconds). If the ignitor is visually intact but not glowing, test for 120V at the ignitor terminals during the ignition sequence with a multimeter. No voltage = control board or ignitor circuit wiring issue, not the ignitor itself.
- 4
Wrong Ignitor Replacement (Resistance Mismatch)
Not all hot surface ignitors are interchangeable. A silicon nitride ignitor installed in a furnace designed for silicon carbide, or vice versa, may not reach proper ignition temperature at the available voltage. Use OEM-equivalent or exact-specification replacement ignitors.
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Quick DIY Checks
Do not work inside the furnace burner compartment with power on except to briefly observe voltage at the ignitor during a live ignition sequence. Gas lines, high-voltage ignitor circuit (120V), and hot surfaces are all present. Always shut off the furnace power before accessing the burner.
Handle hot surface ignitors with extreme care — they are among the most fragile components in the furnace. Never touch the element with bare fingers — skin oils cause failure. Hold only by the ceramic base or mounting bracket.
If gas is smelled during troubleshooting, stop immediately. Ventilate the area, do not operate any electrical switches, and call the gas utility or a certified technician. Do not continue troubleshooting with a gas odor present.
- 1Turn off the furnace power. Open the furnace burner compartment — the ignitor is a small, fragile, gray element mounted near the burner, connected by two wires. Visually inspect the element under a flashlight. Any crack, even a hairline fracture through the glowing surface, means it needs replacement. Do not touch the element with bare fingers.
- 2Disconnect the two ignitor wires at the connector. Set your multimeter to resistance (ohms) mode. Measure across the two ignitor terminals. Silicon carbide: 40–200 ohms is normal. Silicon nitride: 10–30 ohms is normal. OL = open circuit, ignitor is failed. Near-zero ohms = shorted, also failed.
- 3If the ignitor resistance is in range (electrically good), restore power and observe the startup sequence. The control board activates the ignitor for 17–34 seconds before opening the gas valve. Set your multimeter to 120V AC and probe the ignitor wires during this window. You should read 120V. No voltage = control board output is not activating — board issue, not ignitor issue.
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Try Pro — $7.99/mo- 4Order the correct replacement ignitor. Match by furnace brand and model number — using the model number from the furnace data plate guarantees a compatible part. Generic universal ignitors (e.g., Robertshaw 41-409) fit many brands but verify compatibility. When installing, handle the ignitor only by its ceramic bracket — never touch the glowing element.
- 5Install the replacement: remove the single mounting screw, pull the old ignitor out, slide the new one into the mounting bracket, and reconnect the wire harness. Restore power and observe the first ignition sequence. The ignitor should glow bright orange within 15 seconds, gas should ignite, and the flame sensor should detect flame and hold the gas valve open.
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Repair vs Replace
A hot surface ignitor replacement is one of the best-value furnace repairs — $20–$50 in parts, 15 minutes of labor, and the furnace is back in service. This repair makes sense on any furnace regardless of age. Only reconsider if the furnace also has a cracked heat exchanger or failed heat exchanger — then system replacement is more appropriate.
Est. Repair Cost
$20–$50 for a replacement hot surface ignitor
Est. Replacement Cost
$3,000–$6,000 for a new furnace
Recommended Tools & Parts
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Hot Surface Ignitor (universal 120V, silicon nitride or carbide)
Universal replacement hot surface ignitor. Silicon nitride types (longer-lasting, less fragile) fit most modern Carrier, Trane, Lennox, Rheem, Goodman, and York furnaces. Confirm compatibility with furnace model number before ordering.
$20–$50
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Furnace Ignitor (OEM-matched by model)
OEM-exact replacement ignitor sourced by furnace model number. Use the furnace data plate model number to order the exact part. OEM parts guarantee fit and correct resistance specification.
$30–$80
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- How do I know if my furnace ignitor is bad?
- Watch the startup sequence: the furnace control board activates the ignitor for 17–34 seconds. Look into the burner compartment through the observation window — the ignitor should glow bright orange. No glow at all = ignitor is failed (cracked or open circuit). A dim, partial glow from only part of the element = cracked. Test resistance with a multimeter to confirm: silicon carbide should read 40–200 ohms; silicon nitride should read 10–30 ohms. OL = failed.
- What is the difference between silicon carbide and silicon nitride ignitors?
- Silicon carbide (SiC) ignitors are the older design — very fragile, crack easily if touched or thermally stressed, typically gray or dark. Silicon nitride (Si3N4) ignitors are more durable, smaller, and last longer — they are the current standard. Silicon nitride ignitors typically read 10–30 ohms resistance; silicon carbide reads 40–200 ohms. Do not substitute one type for the other without confirming the replacement voltage and heat output spec matches your furnace.
- How long does a furnace ignitor last?
- Silicon carbide ignitors typically last 3–7 years. Silicon nitride ignitors last 7–15 years or longer. Premature failure is almost always caused by physical handling damage (a fingerprint's oil on the element causes a hot spot that cracks it on the first heating cycle) or voltage surges. If your ignitor keeps failing repeatedly, check line voltage — sustained voltage over 125V shortens ignitor life significantly.
- Can I test the ignitor without removing it?
- Yes. With the furnace powered off, access the ignitor wiring connector (usually a 2-wire Molex connector near the burner). Disconnect the connector. Probe the ignitor side with a multimeter in resistance mode. Silicon carbide: 40–200 ohms. Silicon nitride: 10–30 ohms. OL = failed. This test confirms whether the ignitor itself has failed without removing it from the bracket.