Lennox Furnace Error Codes: What They Mean and When to Call
Your Lennox furnace is built to keep you warm through Wisconsin winters, but when something goes wrong, it communicates through a series of fault codes. Whether your unit flashes an LED pattern or displays an alphanumeric code on the control board, knowing what those signals mean can save you time, money, and a cold night at home. This guide covers the most common Lennox furnace error codes, what each one indicates, and exactly when to pick up the phone and call a professional.
How Lennox Furnace Error Codes Work
Lennox furnaces use two methods to report faults. Older models flash a small LED light on the control board in a specific pattern — for example, three flashes followed by a pause indicates one type of problem, while four flashes indicates another. Newer Lennox models with an iComfort or communicating thermostat display alphanumeric codes directly on a screen. Both systems log faults so a technician can review the history even after the error has cleared.
To read LED flash codes: count the number of flashes in the first group, pause, then count the second group. The combination (e.g., 3-4) corresponds to a specific fault. Many Lennox furnace doors have a legend sticker inside that maps these patterns to fault descriptions.
Common Lennox Furnace Error Codes
| Code / Flash Pattern | Meaning | What to Do |
|---|---|---|
| E1 / 1 flash | System lockout — failed to ignite after 3 attempts | Check gas supply; if on, call a technician |
| E2 / 2 flashes | Pressure switch stuck open or failed | Check vent/flue for blockage; call if clear |
| E3 / 3 flashes | Pressure switch stuck closed | Technician required — control board or switch issue |
| E4 / 4 flashes | High-limit device open — overheating | Replace filter immediately; call if problem persists |
| E5 / 5 flashes | Flame sensed without gas valve call | Technician required — possible gas valve fault |
| E6 / 6 flashes | 115V power reversed / polarity issue | Technician or licensed electrician required |
| E7 / 7 flashes | Low flame signal — dirty or failing flame sensor | Homeowner can clean sensor with fine steel wool |
| E8 / 8 flashes | Ignitor failure — no glow or spark detected | Technician required — ignitor replacement |
| E9 / 9 flashes | Rollout switch open — flame spillage detected | Do NOT reset; shut off furnace, call immediately |
| E10 / 10 flashes | Inducer motor failure or speed fault | Technician required — motor or control board |
What Homeowners Can Check Themselves
Air filter: A clogged filter is the number-one cause of high-limit trips (E4). Pull the filter and hold it up to light. If you cannot see light through it, replace it with the correct MERV-rated filter for your model. Check the filter every 30 days during heavy use.
Gas supply: If you see an ignition lockout (E1), verify the gas shutoff valve near the furnace is fully open (handle parallel to the pipe). Also check that your gas service is active if other appliances are also not working.
Vent and flue pipes: A pressure switch error (E2) is often caused by a blocked or disconnected PVC flue pipe. Inspect accessible sections for ice, bird nests, debris, or disconnected joints, especially after a storm.
Thermostat setting: Confirm the thermostat is set to HEAT, the set temperature is above the current room temperature, and the fan setting is AUTO rather than ON.
Reset procedure: For most non-dangerous faults, you can attempt one reset by turning the thermostat off, waiting 30 seconds, and turning it back on. If the same code returns, do not attempt repeated resets — this can cause further damage or mask a safety issue.
When to Call a Professional
Certain codes require immediate professional attention and should never be reset by a homeowner. These include E9 (rollout switch open), E5 (flame sensed with no call for heat), and any code that returns after a single reset attempt. Rollout switches protect against dangerous carbon monoxide spillage — if this trips, shut off the furnace and call for emergency service.
If your Lennox furnace is showing repeated faults, even ones that seem minor, a technician can pull the full fault history from the control board and identify patterns that point to the root cause. Intermittent faults that clear on their own often indicate a component that is beginning to fail.
Burkhardt Heating, Cooling, Plumbing & Electric has served Southeastern Wisconsin for over 60 years. Our NATE-certified technicians are familiar with the full range of Lennox equipment, from older legacy systems to the latest iComfort communicating models.
Schedule Furnace Repair in Southeastern Wisconsin
Don't ignore fault codes — they are your furnace's way of asking for help before a small problem becomes a no-heat emergency in the middle of winter. Whether it's a dirty flame sensor or a failed inducer motor, Burkhardt's technicians carry the parts and expertise to get your Lennox furnace back online fast.
If you see any of these codes, call Burkhardt at 414-206-3049. Our NATE-certified technicians can diagnose and repair the issue — usually in a single visit.
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