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)

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