Lennox Outdoor Unit Control Circuit Open

Lennox air conditioner indoor blower runs but outdoor unit does not; room temperature rises and no heat is removed.

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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

Broken Y call between indoor board and outdoor contactor, Tripped float switch in condensate line, Blown low-voltage fuse on indoor control board, Faulty thermostat cooling relay

How to Fix / Troubleshooting

Safety: Turn off power to indoor and outdoor units before accessing wiring.

  • 1. Confirm thermostat call: Set to COOL and lower setpoint. Verify the thermostat is actually sending a Y call (some Lennox stats show active calls in diagnostics).
  • 2. Check float switch: Inspect condensate pan and float switch. If pan is full, clear drain and reset switch.
  • 3. Inspect low-voltage fuse: Check and replace the 3–5A fuse on the Lennox indoor control board if blown.
  • 4. Verify Y wiring continuity: Ensure the Y wire from thermostat to indoor board and from indoor board to outdoor unit is intact and firmly connected.

Note: If the fuse blows repeatedly, there is a short in the low-voltage circuit that needs professional diagnosis.

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Repair Difficulty

Medium 3/5

Required Part

Lennox Control Board Low-Voltage Fuse
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