Error code reference
Lennox Hvac Error Code 401
The secondary voltage for the air-handler has fallen below 18VAC; if this continues for 10 minutes, the…
The secondary voltage for the air-handler has fallen below 18VAC; if this continues for 10 minutes, the iComfort thermostat will turn off the air-handler.
First step from the service manual
Check the indoor line voltage and transformer output voltage. The alarm clears after the voltage is higher than 20VAC for 2 seconds or after a power reset.
The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.
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Get the full diagnostic procedure for 401.
- Step-by-step tests with expected resistance, voltage, and continuity values
- Wiring diagram references and connector pinouts from the OEM service manual
- Verified part numbers when replacement is required
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