Thermostat shows E201 and reports loss of communication with the indoor unit only; cooling or heating may not respond.

Safety warning: Always shut off power to the furnace/air handler at the breaker and service switch before removing panels.

• 1. Verify indoor unit power:
  • Confirm the furnace/air handler breaker is ON.
  • Check that the blower door is properly closed and engaging the door switch.
• 2. Inspect ABCD or R/C/Y/W wiring:
  • Remove the thermostat from the wall and inspect the communication or conventional terminals.
  • At the indoor unit control board, verify the same wires land on the correct terminals.
  • Look for pinched, cut, or burnt low-voltage wires near the cabinet entry.
• 3. Disconnect accessories temporarily:
  • If a Lennox humidifier, dehumidifier, or air cleaner is wired to the board, disconnect its low-voltage leads.
  • Power cycle the system and see if E201 clears, indicating an accessory wiring fault.
• 4. Check board status LEDs:
  • Observe the Lennox control board LED pattern; if it indicates a board fault, consult the unit's service label.
  • If the board is not powering up or shows communication failure, it may need replacement.
• 5. Call a professional if unresolved:
  • Communication faults often require board replacement and firmware checks; schedule service with a Lennox-authorized technician.