Error code reference
Lennox Hvac Error Code 203
The unit size code for the air-handler has not been selected.
The unit size code for the air-handler has not been selected.
First step from the service manual
Check for proper configuring under unit size codes for air handler on configuration guide or in installation instructions. Remove the thermostat from the system while applying power and reprogramming.
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 203.
- 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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