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Lennox · Air handler

E203 Invalid unit size code

High severity

What the control is telling you

The indoor unit size code has not been set or is incorrect. S30/S40 thermostat alert code 203 (shown as E203 on the unit control display).

Applies to: Lennox communicating air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems

Likely causes

  • Replacement control not programmed
  • Wrong code selected

Test and repair steps

  1. Verify the unit size code against the unit installation instructions.
  2. Remove the thermostat from the system while powering up, then set the unit size code.
  3. Clears after a correct code is read following a reset.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.19. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.