Lennox Furnace Code E203
Hard lockout
The furnace capacity or size has not been programmed into the control. The control does not know what size furnace it is running, so it will not operate.
Manual wording: “Appliance capacity / size is NOT programmed. Invalid unit codes refer to configuration flow chart in installation instructions.”
Documented for: SLP98UHV series
Likely causes, most common first
- New or replacement control board installed without the unit size code being set
- Invalid unit code entered during setup
When to call a technician
This is a setup step done during installation or after a board replacement. A technician has to select the correct furnace size code from the configuration guide.
A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- SLP98UHV Service Literature (Corp. 1029-L7), Table 14 - LED 7 Segment Status / Error Code and Integrated Control Diagnostic Codes (Lennox Industries Inc., SLP98UHV series variable-capacity communicating gas furnace (66,000-132,000 Btuh)) · checked 2026-08-25