Lennox Furnace Light Blinking? How to Decode It
Lennox speaks a couple of dialects. Older and simpler models blink one light in counted bursts. Many Lennox furnaces use two lights together — the combination of which one's flashing, alternating, or steady is the code. A few newer ones have a display that just shows a number. So the first step is figuring out which furnace you've got.
Safety first, every page: smell gas, don't reset anything — leave the house and call your gas company from outside.
Step one: find the legend
On Lennox, more than any other brand, the code chart on the inside of the access panel is the answer key. Pull the lower door's sight line, find the sticker, and match your light pattern to its chart. The patterns below are common meanings, but Lennox varies enough between series that the sticker is the only source that's always right for your unit.
Common Lennox signals
- Both lights blinking slowly in sync — on many models, normal standby. Not a fault.
- Alternating slow blink — commonly "low flame signal": the flame sensor is barely seeing the flame. The furnace may still run today and quit tomorrow. A flame-sensor cleaning is one of the cheapest repairs in HVAC.
- One light steady, other flashing in counts — counted fault codes: commonly pressure-switch problems (check the outside vent pipes for snow or debris — free fix), limit trips (check the filter), or ignition lockouts.
- Rapid alternating flash — frequently ignition lockout: it tried, failed, and gave up. A power cycle often buys one more attempt; if it locks out again, the cause (often that same flame sensor) needs a pro.
Before you call
Write down: the exact light pattern, and the model number (sticker inside the cabinet). Then do the free checks — filter held to a light, vents open, outdoor PVC pipes clear — and one reset: power switch off 30 seconds, on. Same pattern returns? That's your cue to stop resetting and start dialing, armed with the pattern and model number.
Related symptoms
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