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Lennox · Furnace

LED1 & LED2 alternating SLOW flash Watchguard lockout - burners failed to ignite or repeated failures

High severity Lennox G43UF

What the control is telling you

The control locked out. Table 4 lists any of these: burners failed to ignite, limit open for more than 3 minutes (32M88 board only), flame lost 5 times in one heat cycle, or the prove switch dropped out 5 times during one heat cycle.

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

Likely causes

  • No gas supply, wrong line pressure, or gas valve wiring or valve problem
  • Flame sensor loose, broken, grounded or dirty
  • Flue gas recirculating from the termination
  • Burner crossovers damaged or restricted
  • Restricted vent or flaky prove switch (5 prove switch drops)
  • Limit open longer than 3 minutes (32M88 only)

Test and repair steps

  1. Turn off power at the disconnect before touching wiring or components. Gas work: qualified gas tech only.
  2. Check gas line pressure at the valve. It should not go above 13 in. WC. Minimum is 4.5 in. WC natural gas and 8.0 in. WC propane.
  3. Check gas valve and plug wiring. About 35 seconds after the heat call, 24 V should be at the valve. If 24 V is there and the valve stays shut, replace the valve. If no 24 V, replace the control.
  4. Check the flame sensor position, sense wire continuity, and shorts to ground. Clean the rod with steel wool, never sandpaper. Do not bend the rod.
  5. Check that intake and exhaust terminations are not recirculating flue gas.
  6. Pull the burners and inspect crossovers for burrs, warping or blockage.
  7. For the 5 prove switch drops, check the vent for restrictions and test the switch.

Settings for these units

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

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Source: Lennox G43UF Unit Information Corp. 0416-L4, Troubleshooting Chart Problems 4.1, 5.2, 5.4 pp.53-54 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.