Furnace Ignition Systems Diagnosis Reference

Why this matters

Without ignition, the gas valve never opens. The furnace ignition system has evolved through three generations - standing pilot (pre-1990), intermittent pilot / spark (1990-2000), and hot-surface igniter (2000+) - and each has its own failure modes and diagnostic approach. Diagnosing the right one in the first 5 minutes makes the difference between a same-day fix and a return trip.

The three ignition types

Standing pilot (oldest, increasingly rare):

• Small flame burning continuously
• Thermocouple senses the flame; if flame fails, thermocouple cools, gas valve safety closes
• Usually found on 80% AFUE units pre-1990
• Failure: pilot extinguished (drafts, dirty orifice, weak thermocouple)
• Diagnosis: relight pilot, replace thermocouple (inexpensive part) if it won't stay lit

Intermittent pilot (IPI) / spark ignition:

• Pilot lights only when there's a heat call
• High-voltage spark ignites the pilot, pilot ignites the main burner
• Thermocouple or flame rod proves pilot
• 1990s-2000s 80% AFUE furnaces
• Failure: spark electrode fouled / cracked, pilot orifice plugged, ignition module failed
• Diagnosis: visual spark, clean electrode, test for spark gap (1/8" typical)

Hot Surface Igniter (HSI):

• High-resistance ceramic igniter heats to 1800-2400 °F
• Direct ignition of main burner (no pilot)
• 2000+ furnaces (80% and 90%+ AFUE)
• Two material types:
  • Silicon carbide (SiC): older, fragile (the original HSI; cracks with thermal cycling)
  • Silicon nitride (Si3N4): modern, much more durable, lasts 15-20+ years vs 3-7 for SiC
• Failure: cracked igniter (most common), igniter open circuit (winding burned out), wrong-voltage igniter installed
• Diagnosis: ohms test cold, visual inspection

How a hot surface ignition sequence works

1. Thermostat W terminal calls for heat
2. Board energizes inducer; pressure switch closes
3. Board energizes igniter (120 V AC applied)
4. Igniter heats: visible orange glow within 30-60 seconds
5. Board opens gas valve (with igniter still energized)
6. Gas flows past glowing igniter; combustion starts
7. Flame sensor proves flame (microamps); board latches gas valve open
8. Board drops igniter (no longer needed)
9. Blower delay timer starts
10. After ~30-60 sec, blower energizes

If the igniter doesn't glow: open winding, board not sending voltage, wiring break. If it glows but burner doesn't light: gas valve issue, no gas, orifice plugged.

Hot Surface Igniter testing

Visual:

• Look at the igniter through the burner-compartment sight glass during a heat call
• Glowing orange within 30-60 seconds = working
• Glowing red and white (very bright orange) = igniter at full temp
• No glow at all = open winding OR no voltage

Cold ohm test (power off):

• Disconnect igniter wiring at the connector
• Meter on resistance (Ω) scale
• Read across the two igniter leads
• SiC igniter: typically 11-25 Ω cold (varies by manufacturer)
• Si3N4 igniter: typically 40-80 Ω cold (varies)
• Reading: OL (open): igniter winding broken - replace
• Reading: 0 or near 0: shorted - replace
• Reading: within spec: igniter is electrically OK; problem is upstream

Voltage test at the igniter (power on, during ignition trial):

• Disconnect igniter leads
• Meter on V AC, leads to the igniter wires at the harness end
• During ignition trial, should read 120 V AC for ~10-30 seconds
• No voltage = board not sending power → board issue OR safety circuit issue

Spark ignition (older furnaces) testing

Visual:

• Look at the spark electrode during ignition trial
• Should produce strong, bright blue-white spark across the gap (typically 1/8" / 3 mm)
• Yellow / weak spark = high resistance, wet electrode, fouled
• No spark = ignition module issue or wire to electrode

Spark voltage:

• Ignition module produces 10-25 kV for the spark
• Don't try to measure with a regular meter (will damage the meter)
• Verify the cable is intact (no cracks in the silicone-rubber insulation)
• Cracked cable arcs to ground before reaching electrode = no spark at burner

Standing pilot testing

Pilot flame check:

• Pilot should be bluish with yellow tip, 3/4" to 1" tall, engulfing the thermocouple tip
• Yellow / sooty / lazy: dirty orifice, blocked air shutter
• Won't stay lit after holding pilot button: thermocouple weak/failed

Thermocouple test:

• Disconnect thermocouple from gas valve
• Light pilot; hold lit for 30 sec
• Meter on DC millivolts (mV); connect across thermocouple's outer threaded shaft and the inner tip
• Healthy thermocouple: 20-30 mV DC
• Below 12-15 mV: replace

Common failure modes

HSI cracked / open:

• Most common HSI failure
• Often heard before seen: heat cycling causes the ceramic to crack; eventually the crack opens and the winding breaks
• Replacement: modest-cost part, OEM (Si3N4) or aftermarket
• 5-15 min labor

HSI degraded (resistance climbed):

• Older SiC igniters can degrade gradually; resistance creeps up
• Symptom: glow gets dimmer over years; eventually doesn't reach light-off temp
• Replace at PM when resistance is borderline

Spark electrode fouled:

• Carbon buildup on electrode = high resistance = weak spark
• Clean with Scotch-Brite or 0000 steel wool (same as flame sensor)
• Check gap with feeler gauge - typical 1/8"

Pilot orifice plugged (standing pilot):

• Dust, debris, spider webs
• Clean with compressed air or pipe cleaner (don't push debris further in)
• Sometimes orifice needs replacement

Ignition module failed:

• IPI / spark systems
• Symptom: no spark even with verified power
• Replace module

Gas valve sticky / failed:

• Igniter glows but burner doesn't light
• Verify gas is on at the valve handle
• Listen for the click of the valve solenoid when board calls
• 24V test at the valve coil terminals

Diagnostic procedure

1. Read the board's LED code first. Most boards report "ignition failed" with a specific flash code.
2. Visual check during ignition trial:

• HSI: glows? Yes → next step. No → ohms test cold, voltage test at harness.
• Spark: sparks across gap? Yes → next step. No → check module, electrode, cable.

1. If ignition source works but burner doesn't light:

• Verify gas valve handle is ON
• Verify gas line pressure (manometer at the manifold tap, ~3.5" w.c. natural gas typical)
• Listen for gas valve click

1. If burner lights but board doesn't see flame:

• See Flame Sensor Diagnosis and Cleaning Reference

1. Document the trial sequence - when does it fail, in what step? That narrows the diagnosis.

Replacement procedure (HSI)

Time: 20-40 minutes on a typical residential furnace.

Kill power at the service switch and verify dead before opening the burner compartment. Close the manual gas shutoff at the appliance while you work in the burner area.

1. Match the part before you pull the old one. Igniters differ by voltage (line-voltage versus a lower-voltage type on some equipment), by resistance range, by mounting bracket geometry, and by connector. A physically similar igniter with the wrong resistance either burns out fast or never reaches temperature. Cross-reference by furnace model, not by eyeball.

2. Photograph the installed position before removing anything: the bracket orientation, how far the element sits into the burner, and the routing and clipping of the harness. You will want that picture in five minutes.

3. Disconnect the igniter harness at the connector, not by pulling on the leads. Release the retaining clip if there is one. Free the harness from any routing clips.

4. Remove the mounting screw at the bracket and withdraw the igniter straight out. If it does not come freely, stop and look; do not lever it. The ceramic breaks with almost no force.

5. Never touch the ceramic element with bare fingers. Skin oil creates a hot spot that shortens life dramatically. Handle by the bracket or the ceramic base, or wear clean gloves. Do not set it down on the cabinet floor where it can roll.

6. Clean the mounting area. Rust flakes and scale under the bracket change the element's position relative to the burner. Wipe the surface flat.

7. Install the new igniter into the same position the old one occupied, using the photo. Position matters: the element has to sit in the gas stream from the first burner, at the depth the manufacturer specifies. Too shallow and it will not light the burner; too deep and it sits in the flame and cooks.

8. Snug the bracket screw only. Do not torque it. Over-tightening cracks the ceramic base, and the crack often does not show until the first thermal cycle.

9. Reconnect the harness, route it in its original clips, and keep it clear of the burner, the flue, and any sharp edge. A lead that rests against hot metal fails again in a season.

10. Restore gas and power. Run a full ignition trial with the sight glass open. Confirm the element glows within the normal window, the burner lights smoothly across all burners without a rollout or a delayed light-off, and the igniter drops out after flame is proven.

11. Check the flame sensor and microamps while you are here. A furnace that would not light gets diagnosed all the way, and a marginal sensor will bring you back.

12. Watch two or three complete cycles before you close up. Delayed ignition, a soft light-off, or a burner that lights on the far end first points to a gas pressure, orifice, or manifold problem the new igniter did not cause and will not fix.

Also do: check and correct the reason the old one failed. Repeated igniter failure usually means excessive cycling, a cracked or misaligned burner, a bad ground causing weak flame rectification, or a supply-voltage problem. Replacing the part without the cause is a return visit.

References

• ANSI Z21.47 (Gas-Fired Central Furnaces)
• Manufacturer service manuals (Carrier, Trane, Lennox, Goodman, Rheem) - for igniter resistance specs and replacement part numbers
• NATE / RSES training on ignition systems
• Manuall internal: Furnace Pressure Switch Diagnosis, Flame Sensor Diagnosis and Cleaning, Furnace Control Board Diagnosis