Error code reference

Lennox Air Conditioner Error Code E409

Outdoor control secondary voltage is 18VAC or less; after 10 minutes, operation is discontinued.

Outdoor control secondary voltage is 18VAC or less; after 10 minutes, operation is discontinued.

First step from the service manual

Check the indoor line voltage and transformer output voltage.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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E409 on my Lennox Air Conditioner

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Outdoor control secondary voltage is 18VAC or

From Service Manual 873252, p.24:

1.Check the indoor line voltage and transformer output voltage.

2.Measure door lock assembly resistance across the PTC terminals. Expected: ~200 ohms cold, rising sharply once energized.

3.Inspect strike and latch alignment. A warped door or misaligned strike prevents the switch stack from closing in the correct sequence.

Part W10810403

Door lock assembly

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