Hot Surface Igniter Diagnosis and Replacement
The hot surface igniter fails more than almost any other furnace component, and it is one of the easiest parts to misdiagnose or destroy during the swap. Here is how to test HSI resistance, read the right ohm range for the element type, and set a new one without cracking it.
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In This Guide
How a Hot Surface Igniter Works
A hot surface igniter (HSI) is a small ceramic element wired in series with the furnace control board. When the thermostat calls for heat and the pressure switch proves the inducer, the board energizes the igniter. Current flowing through the high-resistance ceramic heats it to somewhere between roughly 1,800°F and 2,500°F — a bright orange-to-yellow glow. Once the warm-up timer expires (usually 15 to 45 seconds), the board opens the gas valve and the glowing element lights the burners.
Understanding the sequence matters because the igniter only lives at one point in the chain. If the pressure switch never closes or the board never calls the igniter, the element is not the problem no matter how it looks. Always confirm you are actually at the ignition step before condemning the part.
The normal ignition sequence
Call for heat → inducer starts → pressure switch proves → igniter warm-up (glow) → gas valve opens → burners light → flame sensor proves flame → igniter de-energizes → blower starts on fan-on delay.
Silicon Carbide vs. Silicon Nitride
The two families behave differently on the meter and in the truck. Get the type right before you read the ohms, or a perfectly good part will look failed and a bad one will look fine.
Silicon Carbide (legacy)
- The classic flat "M" or bar element (Norton 271, White-Rodgers 767A)
- Runs on line voltage, ~115–120V
- Cold resistance typically 40–90 Ω
- Brittle — cracks if you breathe on it wrong
- Shorter life; degrades with every cycle
Silicon Nitride (modern)
- The small round-tip "mini" igniter used in most newer furnaces
- Often runs on 24V (some are 120V — check the label)
- Resistance varies widely by part, commonly 50–400 Ω
- Far more durable and shock-resistant
- Faster warm-up and longer service life
Do not chase a single "good number." Igniter resistance is model-specific. The ranges above are a sanity check, not a spec. Read the value stamped on the OEM part or its bag, and when a universal kit ships with a chart, use that chart. The meter tells you open vs. not open reliably; it only tells you good vs. bad against the correct spec.
Symptoms of a Failing Igniter
A dead HSI usually shows up as a clean lockout: the furnace runs the inducer, waits, and then either does nothing or flashes an ignition-failure code after three or four tries. Watch for these patterns:
- No glow at all during the warm-up window — classic open/cracked element (if voltage is present).
- Weak, dull-orange glow instead of bright yellow-white — a degraded element that may not reach light-off temperature.
- Visible crack or white/gray spot on the ceramic — often a hairline fracture from thermal cycling or a rough previous install.
- Ignites on a hard restart but not cold — a marginal igniter on its way out.
- Ignition-failure fault code (typically the board's slow flash sequence) after the inducer proves.
Glowing but no flame is a different fault
If the igniter glows bright and the burners still never light, the igniter is doing its job. Now suspect the gas valve, gas pressure, a closed manual shutoff, or the board not energizing the valve. A dirty flame sensor causes short flame then lockout, not a no-light — do not blame the igniter for a flame-proving problem.
Testing HSI Resistance (Worked Example)
The ohm test is fast, safe, and settles the question of a cracked element. Do it de-energized.
Scenario
A 90% furnace runs the inducer, waits, and locks out with an ignition-failure code. You see no glow. It uses a silicon carbide (120V) igniter with an OEM cold-resistance spec of 40–90 Ω.
Step 1: Kill power at the furnace disconnect and close the manual gas valve.
Step 2: Unplug the igniter's two-pin Molex from the harness so you read only the element, not the board.
Step 3: Set the meter to ohms (200 Ω range) and touch a lead to each igniter terminal.
Reading: OL (open line)
Step 4: Compare to spec.
OL vs. 40–90 Ω expected → element is open
Step 5: Inspect the ceramic under a flashlight for a hairline crack or a burned-through leg to confirm.
Result: FAILED — replace the igniter
An OL reading with the plug disconnected means the element is broken internally. No amount of voltage will make it glow. Replace it, then confirm the board still delivers voltage before you button up.
Use this quick reference to interpret whatever the meter shows:
| Meter Reading | What It Means | Action |
|---|---|---|
| OL / infinite | Cracked or burned-out element (open) | Replace |
| Within OEM spec | Element intact; not the fault | Test voltage & flame side |
| Far above spec | Degraded / high-resistance element | Replace (likely weak glow) |
| 0 / near zero | Shorted element or shorted leads | Replace; recheck harness |
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Confirming the Board Sends Voltage
If the igniter reads good but never glows, the board may not be energizing it. This test is live, so respect the line voltage. Leave the igniter plugged in and back-probe the harness, or use the board's igniter terminals.
- Set your meter to AC volts.
- Restore power and initiate a call for heat so the sequence runs.
- After the inducer proves the pressure switch, watch the igniter output during the warm-up window.
- A line-voltage HSI should show roughly 115–120V AC; a 24V nitride igniter should show about 24V AC.
Voltage present, igniter good, still no glow?
Recheck your connection at the plug and the ground path. A corroded Molex pin or a poor chassis ground can drop enough voltage to keep a healthy element from reaching temperature. Reseat, clean, and retest before condemning the board.
No voltage during warm-up? The problem is upstream: pressure switch not proving, a limit open, a bad board relay, or the board never advancing. Do not replace the igniter to fix a board that is not calling it. See the related limit-switch and pressure-switch guides below.
Replacing the Igniter Without Cracking It
Silicon carbide elements are ceramic and brittle. The number-one comeback on igniter jobs is a fresh part that cracked during install and read open on the very next cold call. Treat it like glass.
Do
- Handle only the ceramic base or metal bracket
- Match the part by mount, terminal, and voltage
- Seat the mounting gasket flat and clean
- Snug the screws just enough to hold
- Verify the element sits in the flame path, not touching the burner
Don't
- Touch the glow element with bare fingers or tools
- Over-torque the mounting screws
- Force a universal kit into the wrong position
- Let the ceramic contact metal — it will crack when hot
- Reuse a part you dropped, even if it looks fine
Skin oil from bare fingers can create a hot spot that shortens element life, so keep your hands off the glow area. If you use a universal silicon nitride kit as a carbide replacement, follow the kit's harness and mounting instructions exactly — the terminals and gap-to-burner distance are not interchangeable by eye.
Field tip: prove the whole cycle before you leave
After the swap, restore gas and power and watch a full sequence twice from a cold start. You want to see: inducer, a clean bright glow, gas valve open, positive light-off across all burners, flame sense hold, and a normal blower-on delay. One good ignition is not a callback-proof test — cycle it.
Why Igniters Fail Early
Igniters wear out — that is normal. But a part that dies in a season usually has a root cause. If you only replace the element, you will be back. Check for:
- Short cycling from a dirty filter or a nuisance-tripping limit, which multiplies ignition cycles and cooks the element.
- High line voltage pushing a 120V element harder than rated.
- A cracked heat exchanger or flame rollout exposing the igniter to abnormal heat and flame patterns.
- Delayed ignition from low gas pressure or dirty burners, hammering the igniter with a mini-explosion each light-off.
- Vibration from a loose burner assembly or blower, slowly fracturing brittle ceramic.
Fix the cause and the new igniter lasts. Leave the short cycle in place and you have sold the customer a part, not a repair.
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