Lennox Furnace Code 200: Causes, Fixes & Costs
Get this fixed by a local pro
Free, no-obligation quotes from licensed HVAC technicians in your area. Compare up to 3 estimates before anyone visits.
What this code means
On a Lennox furnace, code 200 is an ignition lockout — the control tried to start the furnace, failed to confirm a stable, proven flame the required number of times, and shut the gas off as a safety. It stops attempting and displays the code until power is cycled or it auto-resets after a set delay. The symptom you see is a furnace that tries to start but delivers no heat.
The control isn’t naming a broken part. It’s saying the ignition sequence never completed safely, so it locked out. On these furnaces the most common reason the flame isn’t “proven” is a dirty flame sensor — a thin metal rod that detects the flame’s presence. Coated in oxide, it can’t sense the flame, so the control assumes ignition failed and shuts down. One honest note: the exact code text and retry count vary by Lennox board generation, so confirm the meaning on the diagnostic legend inside your furnace’s blower door. The real work is finding why flame isn’t being proven.
Common causes, ranked by probability
- Dirty flame sensor — by far the most common. Oxide on the sensor rod drops the flame-sense signal below the control’s threshold, so flame is never proven and the furnace locks out after several tries. Burners light, then die in seconds.
- Weak or cracked hot-surface igniter — an aged or hairline-cracked igniter may not reach reliable ignition temperature, causing failed light-offs.
- Restricted gas supply or low pressure — a partly closed utility-side gas valve, a tripped LP regulator, or low manifold pressure starves the burners so flame won’t establish.
- Dirty filter or blocked venting tripping a related switch — restricted airflow or a blocked intake/exhaust can open a pressure switch and stop ignition before it begins, which the control may also lock out on.
- Failed gas valve or control board — the least common cause; concluded only after the sensor, igniter, gas supply, and venting are ruled out.
Safe checks before you call anyone
These are the only steps a homeowner should do on a gas furnace. If they don’t resolve it, the next step is a technician — not deeper disassembly.
- Check the thermostat. Set to Heat, above room temperature, with fresh batteries if it uses them. A failing thermostat can mimic a furnace fault.
- Replace the air filter. A clogged filter restricts airflow and can stop a clean start. Fit a clean filter of the MERV rating Lennox specifies for your model.
- Check the breaker and burner-door switch. Furnace breaker on, and the blower/burner door fully latched so its switch is engaged — the furnace won’t fire otherwise.
- Confirm gas is on elsewhere. If your range or water heater works, house gas supply is fine. Do not open or adjust the furnace’s gas valve yourself.
- Clean the flame sensor (the one allowed internal step). With power off at the breaker and gas off, remove the single screw holding the flame sensor rod, gently buff it with fine emery cloth or 000 steel wool, and reinstall. Don’t touch the burners, igniter gap, gas valve, or any safety switch.
- Cycle power once. After cleaning the sensor or fixing an obvious cause, switch the furnace off for one minute, then back on, and watch one cycle.
If it lights then dies and locks out again after a sensor cleaning, stop here. Repeated lockouts point to an igniter, gas-pressure, or venting problem that needs a technician.
How a technician will diagnose it
Knowing this lets you sanity-check a quote:
- Read the exact code on your board against Lennox’s legend, then clear it and watch a full ignition sequence to see where it fails (no igniter glow, no light-off, or flame that drops out).
- Measure the flame-sense current in microamps with a meter in series with the sensor; a reading below the control’s minimum confirms a dirty or failing sensor.
- Test the hot-surface igniter resistance and confirm it reaches ignition temperature.
- Check manifold and inlet gas pressure against the rating plate, and verify the pressure switch and venting prove correctly.
- Inspect wiring and chassis ground at the control, since flame sensing depends on a solid ground.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Burners light, flame dies in ~5–10s, then code 200 | Dirty flame sensor | Clean sensor rod, retest | Measure flame-sense microamps |
| Igniter never glows, no light-off | Failed hot-surface igniter | — | Test & replace igniter |
| Very dirty filter, weak start | Restricted airflow | Replace filter, retest | Check pressure switch & static |
| Burners try but won’t establish flame | Low/restricted gas pressure | Confirm other gas appliances work | Test manifold & inlet pressure |
| Code 200 returns after sensor cleaning | Igniter, gas, or board fault | Stop, call a pro | Diagnose ignition train end to end |
Repair costs
- Flame sensor cleaning: $0–$15 (DIY, emery cloth only)
- Flame sensor replacement (with diagnosis): $150–$300
- Hot-surface igniter (with diagnosis): $150–$350
- Gas valve replacement: $400–$700
- Integrated control board: $400–$900+ depending on model
A diagnostic service call is typically $89–$200, usually credited toward the repair if you proceed.
Related codes
- Flame-sense / weak flame signal code — flagged before hard lockout when the sensor signal is marginal; same family.
- Watchguard / flame-failure lockout — the lockout state some Lennox boards enter after repeated flame loss.
- Pressure switch fault — an open or stuck pressure switch stops ignition before light-off, an airflow/venting cousin of this lockout.
Parts & tools for this fix
- Furnace flame sensor (match your Lennox model number)
- Fine emery cloth or 000 steel wool (to clean the sensor)
- Furnace air filter (check size on your existing filter)
As an Amazon Associate, fixme.vip earns from qualifying purchases. Prices and availability are on Amazon.
Frequently asked questions
What does Lennox code 200 actually mean?
On Lennox furnaces a code in the 200 range generally indicates an ignition lockout — the control made several attempts to start, could not prove a stable flame each time, and shut the gas off as a safety. The furnace stops trying and shows the code until power is cycled or it auto-resets. The exact code text varies by board, so confirm against your unit's door label, but the condition is a no-heat ignition failure.
Why does my Lennox furnace try to light then lock out?
If the burners light but the flame dies within a few seconds and the furnace eventually locks out, the flame sensor usually isn't proving the flame to the control. A thin oxide film on the sensor rod is the number-one real cause. After several failed proves the board declares an ignition lockout. Cleaning the sensor is the most common fix for this exact pattern.
Can I clear a Lennox ignition lockout myself?
You can reset it once — switch the furnace off at the breaker for a minute, then back on — but only after fixing an obvious cause like a dirty filter or a fouled flame sensor. Don't reset repeatedly. Lockout is a safety state, and forcing more ignition attempts pushes unburned gas into the burner box each cycle. If it locks out again, stop and call a technician.
Is cleaning the Lennox flame sensor safe for a homeowner?
Yes — it's one of the few safe internal fixes. With the power off at the breaker and the gas off, the flame sensor is a single metal rod near the burners held by one screw. Lightly buff it with fine emery cloth or 000 steel wool, never sandpaper, and reinstall. Do not adjust the burners, gas valve, igniter gap, or any safety switch while you're in there.