Lennox furnaces use LED flash codes on the control board to communicate diagnostic information. Here is how to read them.
How to Find the LED
Open the lower furnace panel. The control board is usually visible with a small LED light. Watch the LED pattern — it flashes a certain number of times, pauses, then repeats.
Common Lennox Flash Codes
### 1 Flash — System Lockout (Retries Exceeded)
The furnace tried to ignite multiple times and failed. Reset by turning off the furnace switch for 30 seconds. If it happens again, check the flame sensor, igniter, and gas valve.
### 2 Flashes — Pressure Switch Stuck Closed
The pressure switch should be open when the inducer is off. If it is stuck closed, the switch needs replacement or there is a short in the wiring. Cost: $20-40.
### 3 Flashes — Pressure Switch Stuck Open
The inducer motor is running but the pressure switch will not close. Check for a blocked condensate drain, cracked pressure hose, or failed inducer motor. Check the hose first — it is free to fix.
### 4 Flashes — Open High Limit
The furnace overheated. Check for a dirty filter, blocked vents, or failed blower motor. Replace the filter first. If the blower motor is running slow, its capacitor may be weak.
### 6 Flashes — Rollout Switch Open
Flames are rolling out of the heat exchanger. This is a serious safety issue. The heat exchanger may be cracked. Do not bypass the rollout switch.
### 7 Flashes — Flame Sensed Without Gas Valve Call
The flame sensor is detecting flame when there should be none. Could be a stuck gas valve or faulty flame sensor. This requires immediate attention.
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