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Lennox · Furnace

LED1 OFF / LED2 SLOW flash Prove switch (pressure switch) open

High severity Lennox G43UF

What the control is telling you

The prove switch is not closing with the combustion air inducer running, or it is closed before the inducer starts. Covers blocked intake or exhaust vent too. Shows up as no heat, or burners lighting then shutting off early.

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

Likely causes

  • Prove switch stuck closed before the inducer starts
  • Inducer wiring wrong or loose, or inducer failed
  • Prove switch tubing routed wrong, pinched, broken, blocked or holding condensate
  • Low pressure differential from restricted intake or exhaust vent or wrong vent size or length
  • Wrong concentric vent kit
  • Condensate drain not flowing freely
  • Wrong or out-of-calibration prove switch, or failed switch

Test and repair steps

  1. Turn off power at the disconnect before touching wiring or components.
  2. With the inducer off, confirm the prove switch is open. Replace it if it is stuck closed.
  3. If the inducer does not start, check wiring and look for 120 V at the inducer. Replace the inducer if it has power and will not run.
  4. Check the switch tubing for correct routing, leaks, obstructions and condensate.
  5. Check the pressure differential across the switch. It should be above the set point printed on the switch.
  6. Check the intake and exhaust for blockage, and check vent sizing and length against the installation instructions. Confirm the right termination kit is installed.
  7. Check the condensate line for slope and blockage. It must drain freely while the furnace runs.
  8. Make sure the correct prove switch is installed. Replace it if nothing else is found.

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 2.2, 2.3, 3.1, 5.1 pp.51-53 and Table 4 p.11. Summarized in our own words; check the original manual for your model.

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