Lennox · Furnace

LED1 & LED2 alternating FAST flash Improper ground, broken ignitor, open ignitor circuit or low line voltage

High severity Lennox G43UF

What the control is telling you

Several faults share this pattern: improper main ground (32M88 checks during ignitor warm-up only; 78M47 and 100973-01 check continuously), broken ignitor or open ignitor circuit (32M88), or line voltage below 75 V (32M88) or 90 V (78M47, 100973-01).

Applies to: G43UF-24B-045 through G43UF-60D-135 (SureLight boards 32M88, 78M47, 100973-01)

Likely causes

Furnace not properly grounded
4-pin connector not seated on the board
Line voltage too low
Open ignitor circuit or loose wiring or multi-plug
Broken ignitor (32M88)

Test and repair steps

Turn off power at the disconnect before touching wiring or components.
Check the furnace ground and install a proper main ground if it is missing.
Check the 4-pin connector is fully seated on the board.
Check the line voltage and find why it is low.
Check ignitor circuit wiring and the multi-plug connections.
On 32M88 boards, unplug the ignitor and measure resistance. Replace the ignitor if it is not between 10.9 and 19.7 ohms.

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

G43UF-24B-045G43UF-24B-070G43UF-36B-070G43UF-36C-090G43UF-36C-090HG43UF-48C-090G43UF-48C-110G43UF-48C-110HG43UF-60C-110G43UF-60D-135

Other codes on the same units

LED1 & LED2 alternating SLOW flash Watchguard lockout - burners failed to ignite or repeated failures
LED1 & LED2 simultaneous FAST flash Normal - heat call in progress
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

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

Source: Lennox G43UF Unit Information Corp. 0416-L4, Troubleshooting Chart Problem 1.5 p.51 and Table 4 p.11. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

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.