Lennox Furnace E200 Error
What it means
On a Lennox gas furnace with a digital control board, E200 is a 'Watchguard' hard lockout. In plain terms, the furnace tried to light its burners several times and could not confirm a stable flame, so after exhausting its retries it gives up and locks out to stay safe. Lennox's own definition is broad: Watchguard / E200 means the burners failed to ignite, OR a limit switch stayed open too long, OR the furnace lost flame sense several times in one heating cycle, OR the pressure switch opened too many times. The most common single cause by far is a dirty flame sensor — the thin metal rod that proves to the board that a flame is actually present. When it is coated in residue it cannot 'see' the flame, so the board thinks ignition failed and shuts the gas off.
Common causes of the E200 error
- Dirty or corroded flame sensor rod that cannot detect the flame, so the board assumes ignition failed
- Weak or dirty hot-surface igniter / spark igniter that fails to light the burners reliably
- Low or interrupted gas supply (partially closed gas valve, regulator issue, or empty propane tank)
- Open flame-rollout switch from a blocked burner or blocked heat exchanger
- Restricted airflow tripping the high-limit switch repeatedly (dirty filter, blocked vents)
- Pressure switch opening repeatedly due to a blocked vent, bad hose, or condensate backup
How to fix the Lennox Furnace E200 error
- 1Clear the lockout (power reset)Watchguard often clears itself after a 60-minute wait, but you can reset sooner: turn the thermostat off, switch the furnace power off for about 30 seconds, then back on. Call for heat and watch the ignition sequence. If it lights and runs, you have cleared a one-time fault; if it locks out again, continue below.
- 2Check the air filter and ventsA clogged filter or blocked registers can starve airflow and trip the limit, which feeds into a Watchguard lockout. Replace the filter with the correct size and make sure supply and return vents are open and unobstructed.
- 3Confirm gas is onMake sure the gas shutoff valve at the furnace is fully open, other gas appliances (stove, water heater) work, and — for propane — the tank is not empty. No gas means the burners cannot light, which Watchguard reads as ignition failure.
- 4Inspect (and gently clean) the flame sensorThis is the most common fix but requires opening the burner area, so treat it as advanced. With power and gas OFF, the thin single metal rod near the burners is the flame sensor. A technician (or a confident, experienced homeowner) gently cleans the residue off the rod with fine abrasive and reseats it. If you are not comfortable working near the gas burners, leave this to a professional.
🧰 When to call a professional
Call a licensed HVAC technician if E200 returns after a reset, if you are not comfortable opening the burner compartment, or if you suspect a rollout switch, cracked heat exchanger, failed igniter, or gas-supply problem. A gas furnace involves natural gas or propane, carbon monoxide, and high-voltage electrical hazards, and the E200 lockout sits right at the burner/flame-sensing area. Never bypass safety switches or force the furnace to run. Burner, gas-valve, igniter, and heat-exchanger work should be performed by a licensed professional.
Lennox E200 error — FAQ
What does 'Watchguard' mean on my Lennox furnace?
Watchguard is Lennox's name for a protective lockout after repeated failures. The board tried the full ignition sequence several times, never got a confirmed flame (or kept tripping a safety), and stopped trying so it does not keep dumping gas. E200 is the code for that lockout.
Will the E200 lockout reset on its own?
Yes — Watchguard typically attempts to restart after about a 60-minute reset period. You can also reset it immediately by cycling power. But if the underlying cause (often a dirty flame sensor) is not fixed, it will simply lock out again on the next cycle.
It runs for a minute then shuts off and shows E200 — why?
That pattern strongly suggests a flame-sense problem: the burners light, but the dirty or failing flame sensor cannot prove the flame to the board, so the board drops the gas after a few seconds. Cleaning or replacing the flame sensor (a pro job near the gas burners) usually resolves it.
Sources
This guide is independently written and not affiliated with Lennox. Always unplug appliances before servicing and follow your model's manual. Error codes and steps can vary by model — when in doubt, consult a qualified technician.