Lennox Stuck Contactor or Control Logic Fault

Lennox air conditioner outdoor unit runs in cold weather when it should be off (for cooling-only systems).

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Contactor welded closed, Miswired thermostat or control board, Stuck relay on indoor control board, Thermostat configured incorrectly for heat pump when system is AC-only

How to Fix / Troubleshooting

Safety: Turn off power at the outdoor disconnect immediately to prevent compressor damage in low ambient temperatures.

  • 1. Inspect contactor: With power off, check if the contactor contacts are welded closed. Replace if they do not open freely.
  • 2. Verify thermostat configuration: Ensure the Lennox thermostat is set for cooling-only system if applicable, not heat pump.
  • 3. Check wiring: Confirm Y is only energized when there is a call for cooling, not heating.
  • 4. Replace faulty control board: If the indoor board is sending Y when it should not, it may need replacement.

Note: Running the AC in very cold weather can cause refrigerant migration and compressor damage.

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Repair Difficulty

Hard 4/5

Required Part

Lennox Contactor
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