How to Test a Furnace Hot Surface Igniter (and Why They Keep Failing)
The hot surface igniter (HSI) is one of the most replaced parts on a gas furnace, and one of the easiest to diagnose.
Safety: Igniters get hot enough to burn instantly and carry line voltage during warm-up. Keep hands clear while testing, and turn power off before unplugging it.
Watch the sequence first
On a call for heat, a typical furnace runs the inducer, proves the pressure switch, then powers the igniter for a warm-up period before opening the gas valve. Watch through the sight glass or with the burner door open, as the manufacturer allows:
| What you see | Points to |
|---|---|
| Inducer runs, igniter never glows | Igniter, its wiring or plug, or no voltage from the board |
| Igniter glows dim orange instead of bright | Low voltage at the igniter, or a weak igniter |
| Igniter glows, gas valve never opens | Gas valve or its circuit, or the board; see ignition lockout |
| Igniter glows, gas lights, shuts off in seconds | Flame sensing, not the igniter; see flame sensor readings |
| Inducer runs, nothing else | Pressure switch; see pressure switch troubleshooting |
Step by step
- Read the board's LED code before cycling power.
- Call for heat and wait for the igniter's warm-up period.
- Measure voltage at the igniter plug during warm-up. It should match the manufacturer's spec. No voltage means a board or wiring problem.
- Voltage present, no glow: turn power off, unplug the igniter and measure its resistance. An open reading confirms it's failed. A reading far outside the manufacturer's range points to a failing igniter.
- Inspect it with a light: hairline cracks, white or chalky spots, and broken mounting are all reasons to replace it.
- Check the connector for burned or loose pins.
Silicon carbide vs. silicon nitride
| Silicon carbide | Silicon nitride | |
|---|---|---|
| Look | Gray-black, often a flat or zig-zag element | Usually a smaller, darker, rounded or flat element |
| Durability | Fragile; cracks if bumped or touched | Much tougher |
| Warm-up | Typically longer | Typically faster |
| Replacement | Universal or OEM | Usually OEM or a matched universal; board timing and voltage must match |
The board controls voltage and warm-up time for the igniter it was designed for. A universal igniter must list your furnace model or be rated for the same voltage and type.
Why igniters keep failing
- Touching the element. Skin oils create hot spots that crack it.
- High line voltage. Measure supply voltage; consistently high voltage shortens igniter life.
- Wrong igniter for the board's voltage and timing.
- Short cycling from a high limit or pressure switch problem: every extra ignition attempt wears the igniter.
- Misalignment: an igniter sitting in the flame or too close to the burner overheats.
- Dust and drywall dust after construction.
Replacing an igniter
- Turn off power and gas.
- Remove the burner door and unplug the igniter.
- Remove the mounting screw and slide it out without touching the element.
- Install the new one in the same position, with the same gap to the burner as the original.
- Restore power and gas and watch several full ignition cycles.
Common mistakes
- Replacing the igniter when the board isn't sending voltage.
- Replacing the igniter when the flame sensor is the problem. If it lights, the igniter worked.
- Handling the element with bare fingers.
- Using the wrong type for the board.
- Not finding why the old one failed early.
FAQ
How long should a furnace igniter last?
It varies. Silicon nitride igniters generally last much longer than silicon carbide. Voltage, handling and how often the furnace cycles all affect life.
Can a bad igniter cause a lockout?
Yes. After several failed ignition attempts, the board locks out. See furnace ignition lockout.
Should I carry spare igniters?
Most service techs carry a universal igniter kit plus common OEM silicon nitride igniters for the brands they see most.
Sources
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Field reference for trained technicians. Always follow the equipment manufacturer's instructions, local code and safe work practices. See our editorial standards, or report a correction.