Furnace Won't Ignite - Diagnostic Tree

Symptom

Customer reports the furnace doesn't produce heat. Most common cause: ignition system isn't completing the sequence. Modern furnaces are sequenced - inducer → pressure switch → igniter → gas valve → flame sense → blower. Any step that fails locks out subsequent steps. Reading the LED error code on the control board narrows the diagnosis fast.

Likely Causes

In order of frequency for residential gas furnaces:

• Dirty flame sensor - 25 % of no-heat calls. Rod oxidizes, can't detect flame, gas valve closes
• Failed hot-surface igniter - 15 % (broken or cracked silicon nitride / silicon carbide element)
• Pressure switch fault - 10 % (blocked condensate drain on high-efficiency units, or weak inducer)
• Failed inducer motor - 8 %
• Bad thermostat or thermostat wiring - 8 %
• Tripped safety: flame rollout or limit switch - 7 %
• Closed gas valve or empty propane tank - 6 %
• Failed gas valve (electrical) - 5 %
• Control board failure - 5 %
• Improper ground (modern boards require chassis ground for flame sense) - 4 %
• Other - 7 %

Diagnostic Steps

1. Read the LED error code on the control board. Almost every modern furnace flashes a fault code on a board LED. Decode against the manual or the diagnostic chart on the inside panel.
2. Confirm gas service. Other gas appliances in the home working? Pilot light on water heater lit?
3. Confirm thermostat call. Verify 24V at the W terminal at the furnace (heat call). No 24V = thermostat or wiring issue.
4. Watch a startup attempt. Stand at the furnace, listen + watch:

• Inducer fan starts? Pressure switch closes within 30 seconds?
• Igniter glows orange (hot-surface) or sparks (spark ignition)?
• Gas valve opens and burners light?
• Flame sense confirms within 7 - 10 seconds?
• Blower starts after the heat-exchanger delay?

1. Identify the failed step.

• No inducer start → control board or inducer motor or board-level fuse
• Inducer runs but pressure switch never closes → blocked vent / drain or weak inducer or bad switch
• Pressure switch closes, no igniter glow → failed igniter (check continuity; HSI typically reads 40 - 90 Ω)
• Igniter glows, no gas → bad gas valve, no gas supply, or thermocouple/flame-sense circuit incorrect
• Burners light, lose flame within seconds → dirty flame sensor (the #1 cause)
• Flame stays lit but blower doesn't start → blower limit or board issue

Resolution

• Dirty flame sensor: gently clean the sensor rod with fine emery cloth or a Scotch-Brite pad. Restart. If it lights and stays lit, you're done.
• Pressure switch fault: trace the hose for kinks, clear condensate drain, verify inducer pressure. Replace switch if pressure is correct but switch doesn't close.
• Failed inducer: replace assembly.
• Empty propane tank: customer fix; call propane supplier.
• Stuck-closed gas valve (electrical): verify 24V at the valve when board commands open. No power = board issue; power present, no flow = valve dead.

Prevention

• Annual maintenance catches a marginal flame sensor + dirty burners before the no-heat call
• Flame-sensor cleaning is a 2-minute checklist item that prevents 25 % of next year's calls

References

• NFPA 54 (National Fuel Gas Code)
• Manufacturer service literature for the specific furnace
• Manuall internal: Furnace Annual Maintenance, AC Won't Cool - Diagnostic Tree