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Error code reference
Lennox HVAC Error Code 203
The unit size code for the air-handler has not been selected.
SourceLennox CBX40UHV - Service Manual: Installation & Service (HFC-410A)p. 35
Diagnostic procedure
The full 203 procedure
Step 1 is visible so you can confirm you are on the right page. The remaining steps, the ones that tell you whether to actually replace a part, unlock with any pass.
MyPros+ · DiagnosticsStep 1 preview
1
First diagnostic step
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.
SourceLennox CBX40UHV - Service Manual: Installation & Service (HFC-410A)p. 35
More steps, wiring diagram references, and verified part numbers
Includes expected resistance and voltage values, connector pinouts, and supersession notes for replacement parts.
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Where this code is documented
1 manual in the MyPros+ library documents this code for Lennox hvac.
Models covered by the manuals that document this code
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