HVACopediaLennox · Furnace
LED1 & LED2 simultaneous FAST flash Normal - heat call in progress
What the display is showing
Both LEDs flash fast together (about 3 flashes per second) when the thermostat starts a heat call. Normal operation.
Applies to: G43UF-24B-045 through G43UF-60D-135 (SureLight boards 32M88, 78M47, 100973-01)
Settings for these units
- Recommended filter size by cabinet width · EL296UHV
- Recommended filter size by cabinet width · Lennox SL280UHV
- Filter sizes by cabinet width · Lennox ML180UHE
- Filter sizes (filter rack kits) · Lennox EL195UHNE
- Filter sizes by cabinet width · Lennox SL297NV
Models that show this code
10 models in Lennox G43UF
Other codes on the same units
- LED1 & LED2 alternating FAST flash Improper ground, broken ignitor, open ignitor circuit or low line voltage
- LED1 & LED2 alternating SLOW flash Watchguard lockout - burners failed to ignite or repeated failures
- LED1 & LED2 simultaneous SLOW flash Normal - power on, idle, cooling or continuous fan
- LED1 FAST flash / LED2 SLOW flash 120 V line and neutral reversed
- LED1 OFF / LED2 OFF No power to the control
- LED1 OFF / LED2 ON Control board failure or control wired incorrectly
- LED1 OFF / LED2 SLOW flash Prove switch (pressure switch) open
- LED1 ON / LED2 OFF Control board failure or control wired incorrectly
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Source: Lennox G43UF Unit Information Corp. 0416-L4 (rev 2-2006), Table 4 Diagnostic Codes p.11. Summarized in our own words; check the original manual for your model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.