Lennox air conditioner will not start; thermostat calling for cooling but outdoor unit is completely dead, no fan, no compressor, no click from contactor.

Safety first: Turn off power at the outdoor disconnect and the indoor air handler/furnace breaker before opening any panels. Use a non-contact voltage tester to confirm power is off.

Step-by-step checks:

• 1. Verify thermostat operation: Set thermostat to COOL and lower the setpoint at least 3–4°F below room temperature. Confirm the display is on and batteries (if used) are good.
• 2. Check low-voltage fuse: At the indoor Lennox control board (in furnace/air handler), locate the 3–5A blade fuse (often purple). Inspect for a blown element. Replace with the same rating only. If it blows again, there is a short in the low-voltage wiring.
• 3. Inspect float/condensate safety switch: If your Lennox system has a float switch on the condensate drain or pan, ensure the pan is not full. Clear the drain line and reset the float switch. A tripped float switch will open the 24V circuit and prevent cooling.
• 4. Check thermostat wiring: At the control board, verify R and Y wires are firmly connected and not corroded. At the outdoor unit, verify the low-voltage wires from the house are tight on the contactor coil terminals.
• 5. Test 24V transformer (advanced): With power restored and extreme caution, measure between R and C at the control board. You should see ~24VAC. If there is no 24V output but the primary side has 120VAC, the transformer is likely failed and should be replaced by a qualified technician.

When to call a pro: If the fuse repeatedly blows, or you cannot safely test 24V circuits, contact a licensed HVAC technician to locate shorts in the thermostat cable or control wiring.