HVACopediaLennox · Furnace
LED1 SLOW flash / LED2 FAST flash Low flame signal
What the control is telling you
Flame is proven but the signal is weak. Trips below 0.18 microamps on board 32M88, and below 1.5 microamps on boards 78M47 and 100973-01. The furnace still runs. Low flame current is rated at 0.16-0.17 microamps (32M88) or 0.5-1.4 microamps (78M47, 100973-01).
Applies to: G43UF-24B-045 through G43UF-60D-135 (SureLight boards 32M88, 78M47, 100973-01)
Likely causes
- Flame sensor rod positioned wrong on the burner
- Flame sensor rod dirty or coated
Test and repair steps
- Turn off power at the disconnect before touching wiring or components.
- Check that the sensor rod sits correctly in the flame. Reposition it, or replace it if it cannot be positioned.
- Clean the rod with steel wool. Do not use sandpaper. Replace the sensor if cleaning does not help.
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 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
- LED1 ON / LED2 ON Control board failure or control wired incorrectly
- LED1 ON / LED2 SLOW flash Rollout switch open
- LED1 SLOW flash / LED2 OFF Flame sensed with gas valve not energized
- LED1 SLOW flash / LED2 ON Primary or secondary limit open
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Source: Lennox G43UF Unit Information Corp. 0416-L4, Troubleshooting Chart Problem 6 p.55 and Table 4 p.11. Summarized in our own words; check the original manual for your model.
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