Most igniter swaps fail to fix the furnace for one of three reasons: the igniter was never the problem, the replacement doesn't match what the control board expects, or whatever killed the first igniter is still there. This guide is about avoiding all three, so the new part is the last one you buy for years.
Prove it's the igniter before you buy one
On a typical induction-draft furnace the board runs a fixed sequence: the thermostat calls for heat, the inducer fan starts, the pressure switch closes to prove draft, the igniter warms up (commonly 15 to 45 seconds depending on the board), the gas valve opens, and the flame sensor must confirm flame within a few seconds. Where the sequence stops tells you which part to suspect.
The most common misdiagnosis is the second row. Because the board only powers the igniter after the pressure switch closes, a failed or blocked pressure switch looks identical to a dead igniter. Read the blink code on the board's LED through the door window before you order anything. A pressure switch fault code means a new igniter changes nothing.
The cleanest test is voltage, not resistance. With power on, the thermostat calling, and the igniter unplugged from its harness, put a meter on the harness side of the connector during the warm-up window. A 120 V igniter circuit should show roughly line voltage. Voltage present and a dead igniter means the igniter is bad. No voltage means the problem is upstream: pressure switch, limit, board. Some boards ramp the voltage up gradually, so read near the end of the warm-up period.
Resistance checks are weaker than people think. A hairline crack can conduct fine when the igniter is cold and open up as the ceramic expands with heat, so a normal cold reading does not clear it. A visible crack, a chip near the hot zone, or an open circuit does condemn it. Silicon carbide igniters also drift upward in resistance as they age, which means less current, a lower element temperature, and slower ignition well before total failure.
Know which type you have
Two materials dominate, and they behave differently enough that swapping one for the other is not always harmless.
The reason this matters: the board is written around one igniter's heat-up curve and current draw. Universal replacements work in many furnaces, but where the board ramps power, or where a different element runs hotter or slower than the original, you get weak ignition or an igniter that cooks itself early. When possible, match the original part number from the furnace data plate and the existing igniter, then match voltage, connector style, bracket pattern, and element length. Never assume a plug that fits is a part that's correct.
Work out why the first one failed
If the old igniter lasted many years, age was the cause and you can move on. If it died in under two or three years, something is shortening the life of every igniter this furnace gets.
The usual causes, in the order worth checking:
Short cycling. Every call for heat is one full heat and cool cycle for the ceramic. A clogged filter that trips the high limit, an oversized furnace, or a thermostat in a bad spot can multiply the cycles per day. Watch a few cycles and count how long the burners actually stay on.
A degraded connector. The plug between the igniter and the harness carries the full warm-up current. If the terminals are brown, black, or the plastic is softened, resistance builds heat at the connector, the igniter gets less power, and the next one fails the same way.
Replace the harness pigtail along with the igniter.
Voltage above rating. Persistently high line voltage shortens element life. Measure at the igniter during warm-up, not at the outlet.
Skin oil or contamination. Fingerprints on the hot zone create local hot spots that crack the element.
Vibration or impact. A loose burner box, a mounting screw over-tightened onto the ceramic, or a bump during an earlier service.
Moisture and debris. On high-efficiency furnaces, condensate or flue moisture reaching the burner area, plus rust flakes or lint falling on the element.
Replacement, step by step
Turn off furnace power at the switch and the breaker. Close the gas shutoff so the handle sits crosswise to the pipe. Let everything cool, since a hot igniter assembly and a hot burner box both cause burns.
Photograph the wiring, bracket, and igniter position before touching anything.
Remove the door and the burner compartment cover.
Unplug the igniter by gripping the connector body, not the wires.
Remove the mounting screws or clip holding the bracket.
Hold the old and new igniters side by side and compare element length, bracket holes, and lead length.
Handle the new one by the base or the ceramic body, never the element tip. If you do touch it, wipe it with isopropyl alcohol and let it dry fully.
Mount it where the original sat. The hot zone has to sit in the gas stream near the burner ports. Follow the original position from your photo, or the spec sheet if it's a universal kit.
Snug the screws only. Ceramic cracks under clamping stress, and this is a common way a brand-new igniter dies within a week.
Route the leads away from the gas manifold, sharp sheet metal, and anything that gets hot.
While the compartment is open, clean the flame sensor with a fine abrasive pad (not coarse sandpaper, which leaves grit and can scratch the rod), check the burner ports for lint and rust, and vacuum out debris. If the igniter died of old age, the sensor is probably the same age and close to its own failure.
Check the result properly
Restore gas and power, close the panels so the door switch engages, and run the furnace through at least three full cycles while watching through the sight glass. The igniter should go from dull orange to bright orange or yellow-white, and the burners should light within a few seconds of the gas valve opening. A soft whump or delayed ignition means gas is collecting before it lights, which points to element position, a weak igniter, or dirty burners. Check that your carbon monoxide detectors are working, and smell around any fitting you disturbed.
If you have a meter with a microamp DC range, measure flame sensor current in series with the sensor lead. Boards generally need somewhere around 1 microamp or more to hold the flame, and a healthy sensor often reads a few microamps. A marginal sensor can pass on a clean furnace and then start tripping a few weeks later, and that looks exactly like the new igniter being bad.
After repeated failed ignition attempts most boards lock out, often for about an hour. Cycling power at the furnace switch resets it. If it locks out again immediately, stop and diagnose instead of resetting repeatedly.
When to stop and call a technician
You smell gas, or see soot, scorch marks, or melted wiring around the burner box.
Flames are lazy, yellow, or rolling out of the burner compartment.
The igniter glows correctly and the board throws no fault, but gas never flows.
You suspect the heat exchanger is cracked.
Anyone suggests jumping out the pressure switch or a limit switch to get the furnace running. Those switches protect you from carbon monoxide and fires, so don't defeat them.
If you're in the Philadelphia area and require furnace replacement services Cool Aid Co. can find out what it is.
What to expect on cost
Rough U.S. figures: aftermarket igniters tend to run about $15 to $50, OEM parts about $40 to $150, and a service call with the part commonly lands between $150 and $400. Pricing varies by region and brand. Also check whether the furnace is still under manufacturer parts warranty, because many cover the igniter for the first several years and the serial number on the data plate tells you the install date.
