Lennox E313 Error Code

What does Lennox E313 mean?

E313 is a warning-level alert on Lennox communicating systems indicating the indoor and outdoor unit capacity codes do not match. The system continues to operate normally — no lockout occurs. The mismatch is detected during commissioning when the indoor unit size code configured in the control does not match the capacity reported by the outdoor unit. The alert clears automatically when commissioning is exited.

Symptoms

• E313 or capacity mismatch notification shown during system commissioning.
• System operation is not impacted — heating and cooling continue normally.
• Alert appears at system startup or after a thermostat or control replacement.
• May recur after any component replacement that resets the unit size code.

Common causes

• Incorrect indoor unit capacity/size code selected during initial commissioning.
• Outdoor unit replaced with a different tonnage without updating the indoor unit code.
• Indoor control board replaced and unit size code not reconfigured.
• Thermostat or communicating control reset clearing the stored configuration.

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Diagnostic steps

1. Identify the correct indoor and outdoor unit capacity codes

   Locate the model number on the rating plate of both the indoor unit (furnace or air handler) and the outdoor unit. Cross-reference the capacity/size code in the Lennox installation instructions for the indoor unit.

2. Access the unit size code configuration menu

   On the Lennox communicating thermostat, navigate to the dealer/installer configuration menu. Locate the indoor unit size code setting and compare it to the value required by the installation instructions.

3. Set the correct unit size code

   Enter the correct capacity code for the installed indoor unit. For the SLP99UHV, refer to the configuration table in the installation instruction or service literature. If the thermostat is removed during configuration, follow the procedure in the indoor unit installation instructions.

4. Exit commissioning mode and confirm the alert clears

   After setting the correct code, exit the commissioning or configuration mode. E313 should clear automatically. If it does not, cycle power and verify the stored code matches the nameplate value.

When to call a professional

E313 is a configuration issue, not a mechanical failure. If you are not familiar with the Lennox communicating thermostat dealer configuration menus or do not have access to the dealer PIN, a licensed Lennox dealer must perform the unit size code correction. Incorrect capacity coding can affect system efficiency and variable-capacity operation, so confirm the correct code with Lennox technical support if unsure.