Lennox furnace Alert code 203: Invalid or unset unit size code
Alert 203 means the unit size code for the indoor unit has not been selected, or has been set incorrectly. Without it the control does not know what equipment it is installed in.
Applies to: Lennox communicating gas furnaces and air handlers on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
This is the companion to the size mismatch alert, and the distinction is that here the code has not been set at all or is invalid, rather than being set to something that conflicts with the motor. Lennox's remedy is the same procedural sequence: remove the thermostat from the system while applying power, then set the unit size code per the instructions provided in the indoor unit installation instruction. The codes themselves are listed in the system component configuration guide or the installation instructions, which means the correct value is model-specific and cannot be guessed.
Most common causes
- The unit size code never set during installation.
- An incorrect unit size code entered.
- A replacement control fitted without configuration.
- Configuration lost after a control replacement or reset.
How to diagnose it
- Establish whether the system has just been installed, since an unset code most often means installation was not completed.
- Establish whether a control has been replaced recently.
- Have the indoor unit's model number available, since the code is model-specific.
- Note that Lennox says the alert clears automatically once the correct match is detected following a reset.
How to fix it
- This is technician work following Lennox's documented procedure.
- Lennox's instruction is to remove the thermostat from the system while applying power and set the unit size code per the indoor unit installation instruction.
- If the system was recently installed, call the installer — configuration is part of the installation.
- Do not leave the code unset. The control uses it to determine how to operate the equipment.
Notes
An unset configuration code on a newly installed system is an incomplete installation rather than a fault. It is worth framing it that way when calling, because it is the installer's work to finish rather than a warranty repair.
Related
- Alert code 202 — Blower motor and unit size mismatch
- Alert code 201 — Blower motor communication lost
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.