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Lennox · Air conditioner

12 Indoor unit not detected

High severity

What the control is telling you

The thermostat cannot find a Lennox communicating indoor unit (furnace or air handler) on the system. S30/S40 thermostat alert code 12.

Applies to: Lennox communicating air conditioners incl. variable-capacity inverter units in S30/S40 smart thermostat systems

Likely causes

  • No power to indoor unit
  • Open or loose R, i+, i-, C wire at mag-mount/subbase, smart hub or indoor control
  • EIM not configured as furnace or air handler (non-communicating indoor unit)
  • Failed indoor control

Test and repair steps

  1. Check for 24 VAC at the indoor control and look for a missing component.
  2. Check R, i+, i- and C at the mag-mount or subbase, smart hub and every communicating device; ohm the wires for continuity.
  3. Cycle power to the indoor unit first, then the thermostat.
  4. With a non-communicating indoor unit, make sure the equipment interface module is configured as air handler or furnace.
  5. S30 go to Menu > Settings > Advanced Settings, open View Dealer Control Center, then pick Equipment > Reset All Equipment. The thermostat will find every Lennox communicating device again.
  6. S40 go to Menu > Settings > Advanced Settings, open View Service Support Center, then Equipment Setting > Reset and pick Reset HVAC Equipment.
  7. Replace the indoor control if it still does not respond. Clears automatically once fixed.

Settings for these units

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.9. Summarized in our own words; check the original manual for your model.

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