Lennox Furnace Won't Ignite: Causes, Fixes & Costs
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Lennox furnace won't ignite: what it means
“Won’t ignite” is a symptom, not a single Lennox fault code — it describes a furnace that calls for heat, often runs the inducer and igniter sequence, but never lights the burners (or lights briefly and drops out).
Lennox unit controllers report faults differently depending on the board generation: older boards use an LED flash pattern, while newer SureLight and iComfort-era boards show a numeric alert code on a seven-segment display or push it to a communicating thermostat. We won’t guess at your code number. Read the code or flash pattern off your own board (or thermostat alert screen), write it down, and look it up in the installation/service manual for your exact unit — the manual is usually in a sleeve inside the blower door, and the model number is on the rating plate.
Retry and lockout behavior also depends on the control board. Some Lennox controls make a fixed number of ignition attempts and then hold the lockout until power is cycled; others enter a Watchguard-style state and retry automatically after a timed delay. Don’t assume a particular number of attempts or a particular reset method — your unit’s manual states what that specific board does. The practical takeaway is the same either way: one power cycle is a reasonable test, and a furnace that locks out again has a hardware or gas-supply fault that needs a technician.
Lennox furnace won't ignite: common causes, ranked by probability
- Failed hot-surface igniter — the most common ignition failure; the igniter cracks or no longer glows hot enough to light gas.
- Dirty or failed flame sensor — the furnace lights but can’t “prove” flame, so it shuts the gas within seconds and retries, then locks out.
- Closed or interrupted gas supply — a shut-off valve, tripped excess-flow device, or utility interruption means no gas reaches the burners.
- Pressure switch not proving — a blocked flue/intake, condensate blockage, or weak inducer prevents the pressure switch from closing, so the control never allows ignition.
- Gas valve fault or wiring — a failed valve or loose valve wiring means gas isn’t released on the ignition call.
- Control board / ignition sequence fault — a board that mis-times or fails to energize the igniter or valve.
- Open limit or rollout switch — a safety switch holding the sequence open. ⚠️ Safety: this is the least likely cause on the list but the most serious. A tripped rollout switch means flame or hot combustion gas came out of the burner compartment instead of going up the flue — a cracked heat exchanger, blocked flue, or burner problem. Do not reset the furnace and do not run it. Shut the furnace off at its switch, leave it off, and have a technician inspect it before it runs again. If you smell gas or suspect combustion products in the house, leave and call your gas utility or 911.
How to fix Lennox furnace won't ignite: safe checks first
These are the only steps a homeowner should attempt:
- Thermostat: Confirm it’s set to Heat and the setpoint is above room temperature. Replace the batteries if it’s battery-powered.
- Air filter: A severely clogged filter can trip limits and disrupt the heat cycle. Replace a dirty filter.
- Breaker / furnace switch: Check the furnace breaker and the wall switch (looks like a light switch) near the unit. Reset a tripped breaker once.
- Gas appliances: If your gas stove or water heater also won’t work, the gas supply may be off utility-wide — contact your gas provider.
- Vents and registers: Make sure supply and return registers are open and unobstructed.
- Condensate drain (high-efficiency models): Look at the external drain line and the area under the furnace for standing water or an obvious overflow, and check that the condensate pump (if you have one) is plugged in and running. Report what you see to the technician. The condensate trap is inside the cabinet — leave it alone.
- One reset: Cycle the furnace switch or breaker off, wait a minute, and back on — once. If it still won’t ignite, stop and call a pro. (Skip this entirely if you have any reason to suspect a rollout trip — see cause 7 above.)
Do not open the furnace cabinet, touch the igniter, flame sensor, condensate trap, gas valve, or wiring, or reset the unit repeatedly. Those are technician tasks.
How a technician will diagnose it
A qualified tech will typically:
- Read the stored fault code and flash history on the control board and cross-reference it to the manual for that model.
- Watch a full ignition sequence to see where it fails (inducer, igniter glow, valve opening, flame proving).
- Measure the hot-surface igniter resistance and confirm it glows and reaches temperature.
- Check flame current (microamps) with a meter and clean or replace the flame sensor.
- Verify gas is present and measure manifold and inlet gas pressure.
- Test the pressure switch, inducer, and vent/intake for blockage; inspect the condensate trap and drain.
- Check limit and rollout switches and the gas valve coil/wiring, and investigate the combustion cause behind any rollout trip.
Cross-check any quote against the specific code the board reports — a blanket “replace the control board” without these tests is a red flag.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Igniter glows, no flame | Closed gas valve or faulty gas valve | Confirm other gas appliances work | Test gas pressure, valve coil, wiring; replace valve |
| Igniter doesn’t glow | Failed hot-surface igniter | None — cycle power once | Test igniter resistance; replace igniter |
| Lights then shuts off in seconds | Dirty/failed flame sensor | None | Measure flame current; clean or replace sensor |
| Inducer runs, no ignition | Pressure switch won’t prove / blocked vent | Check for obvious vent blockage or condensate overflow | Test pressure switch, inducer, clear flue and drain |
| No sequence at all | Open limit/rollout or control fault | Replace filter; reset breaker once — not if rollout is suspected | Test safeties, find the combustion cause of any rollout trip, inspect board |
| Repeats then stops trying | Ignition lockout after retries | One power cycle only | Diagnose root cause before resetting |
Lennox furnace won't ignite repair cost
Honest US ranges, parts and labor:
- Hot-surface igniter: $150–$350
- Flame sensor clean: $90–$180; replacement: $150–$300
- Pressure switch: $150–$350
- Inducer motor: $400–$1,200
- Gas valve: $350–$600
- Control board: $400–$1,200 — Lennox communicating (Signature-series) boards sit at the top of that range and can reach roughly $1,500 installed, so treat the upper end as open-ended and get a written part number and price before approving the job
- Diagnostic / service call: $90–$180 (often credited toward repair)
- Shut gas valve or clogged filter: $0 if you catch it yourself
Costs vary by region, model, and parts availability. Heat-exchanger or flue repairs found behind a rollout trip are a separate, larger job.
Related pages
- Lennox Furnace Ignition Lockout: Causes, Fixes & Costs
- Lennox Furnace Flame Sensor Problems: Fixes & Costs
- Lennox Furnace Clicking But Not Starting: Causes & Fixes
- Looking up your own code: if your board or thermostat shows a numeric alert code or LED flash pattern, match it to the fault table in the installation manual for your exact Lennox model rather than to a generic code list.
Parts & tools for this fix
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Frequently asked questions
Why won't my Lennox furnace ignite but the blower still runs?
The control may run the inducer and blower while failing ignition — usually a failed hot-surface igniter, dirty flame sensor, closed gas supply, or a pressure switch that won't prove. These are technician repairs.
Can I reset a Lennox furnace that won't ignite?
You can cycle power at the switch or breaker once to clear a lockout. If it fails to ignite again, stop and call a pro — repeated resets won't fix a hardware fault and can be unsafe.
How much does it cost to fix a Lennox furnace that won't ignite?
A hot-surface igniter runs about $150–$350 installed, a flame sensor clean $90–$180, and a gas valve $350–$600. A control board is roughly $400–$1,200, and a Lennox communicating board can run to about $1,500 installed. A gas supply issue may cost nothing if a valve was simply shut.