Thermostat displays E200 and cannot communicate with the indoor unit or outdoor unit; system may not heat or cool.

Safety first: Turn off power to the HVAC system at the breaker panel and at the furnace/air handler service switch before touching any wiring.

• 1. Power-cycle the system:
  • Turn off the furnace/air handler and outdoor unit breakers for 2–3 minutes.
  • Turn power back on and wait for the thermostat to reboot and attempt to reconnect.
• 2. Check thermostat wiring:
  • Remove the Lennox smart thermostat from its wall base.
  • Inspect the R and C (or ABCD) terminals for loose or broken wires.
  • Ensure each conductor is fully inserted and tightened under the terminal screw.
• 3. Inspect communication bus (ABCD) on Lennox communicating systems:
  • At the furnace/air handler control board, locate the ABCD terminals.
  • Verify that A-A, B-B, C-C, D-D are color-matched and consistently wired from thermostat to furnace to outdoor unit.
  • Tighten any loose terminal screws and correct any crossed wires.
• 4. Check low-voltage fuse and 24V power:
  • With power off, inspect the 3A or 5A blade fuse on the Lennox control board; replace if blown.
  • Restore power and confirm the board LED is on and 24V is present between R and C.
• 5. If E200 persists:
  • The issue may be a failed Lennox furnace/air handler control board or a failed communication module in the thermostat.
  • Contact a qualified Lennox technician for board-level diagnostics and replacement.