Thermostat intermittently reboots or shows E211 low power warning, especially when equipment starts.

Safety warning: Turn off power to the indoor unit before tightening any connections.

• 1. Inspect and tighten low-voltage connections:
  • At the Lennox control board, tighten R and C terminal screws.
  • Check any wire nuts splicing the thermostat cable and re-make loose connections.
• 2. Reduce accessory load:
  • Temporarily disconnect non-critical 24V accessories (e.g., UV lights, non-Lennox add-ons) from the board.
  • See if the low power warning disappears.
• 3. Check cable gauge and length:
  • If the thermostat run is very long (>100 ft) and uses small-gauge wire (e.g., 22 AWG), consider upgrading to a heavier gauge cable.
• 4. Have transformer capacity checked:
  • A technician can measure loaded voltage and current draw on the 24V transformer.
  • If the transformer is undersized or failing, it should be replaced with a Lennox-approved part.