Compiled & reviewed by Nikolai Tsyrenov · Updated 2026-06-25
How do I read Lennox furnace error codes?
Lennox furnaces display faults either as a counted red-LED flash code on older boards or as an 'E###' alphanumeric code on the newer seven-segment LED (and on iComfort/communicating thermostats, where it shows as the bare number).
How to read Lennox furnace codes
Lennox furnaces display faults either as a counted red-LED flash code on older boards or as an 'E###' alphanumeric code on the newer seven-segment LED (and on iComfort/communicating thermostats, where it shows as the bare number). On flash-code boards a slow one-second heartbeat means normal idle; a rapid flash repeated a set number of times is the code. On seven-segment boards, letters like 'H' (gas heat stage) or 'C' (cooling) are NORMAL status indicators — only 'E###' entries are faults. Flash-code meanings are board-specific, so confirm against the chart on your furnace's access panel.
⚠️ Safety first. Gas furnaces burn fuel and produce combustion gases. If you ever smell gas (a rotten-egg or sulfur odor), hear hissing, or your carbon-monoxide alarm sounds, do not touch light switches, thermostats, or the furnace — leave the home immediately and call your gas utility's emergency line or 911 from outside. Homeowners should limit themselves to simple, safe steps: setting the thermostat, checking and replacing the air filter, confirming the furnace switch and breaker are on, keeping supply registers and return grilles open, and clearing snow, ice, or debris from outdoor vent and intake terminations. Never bypass, jumper, or reset safety switches (limit, flame-rollout, or pressure switches), never open the gas valve or burner compartment, and never attempt combustion, electrical, or control-board repairs yourself. Error codes point toward a problem but are not a substitute for proper diagnosis — when in doubt, turn the system off and call a licensed HVAC technician. Keep working carbon-monoxide detectors on every level of your home.
Lennox furnace code FAQ
How do I read the error code on a Lennox furnace?
Lennox furnaces display faults either as a counted red-LED flash code on older boards or as an 'E###' alphanumeric code on the newer seven-segment LED (and on iComfort/communicating thermostats, where it shows as the bare number).
Where is the diagnostic light on a Lennox furnace?
It's on the control board, usually visible through a small sight glass on the lower blower-compartment door. You may need to remove the door panel to read it clearly.
What is the most common Lennox furnace code?
Code E250 — Primary limit switch circuit open — the limit-circuit safety has tripped, typically from overheating due to low airflow. If the limit doesn't close within 3 minutes the unit enters a 1-hour soft lockout (Watchguard). On older red-LED boards this same condition is 7 flashes.