Thermostat displays E1 and system will not heat or cool; often indicates indoor unit not detected

Safety: Turn OFF power to the indoor unit at the breaker and service switch before removing panels.

• Confirm indoor unit power: Ensure the Ruud furnace/air handler has power (service switch ON, breaker not tripped). Check for any status LEDs on the control board.
• Check ABCD connector: Remove the blower door and locate the ABCD communication terminals or plug on the Ruud control board. Make sure the thermostat cable is firmly attached and wires are not broken or corroded.
• Verify configuration: Some Ruud indoor boards have configuration dip switches or jumpers for system type. Compare switch positions with the installation manual to ensure the board is set for the correct equipment (gas furnace, air handler, heat pump, etc.).
• Inspect low-voltage fuse: With power OFF, remove and test the low-voltage fuse on the indoor control board. Replace if blown and investigate for shorts in thermostat or accessory wiring.
• Reboot system: After checking connections, restore power to the indoor unit and thermostat. Allow several minutes for the EcoNet network to reinitialize. See if the E1 code clears and the indoor unit appears in the thermostat equipment list.
• Board failure: If the thermostat still cannot detect the indoor unit despite correct wiring and power, the indoor control board's communication circuit may be defective and should be replaced by a qualified technician.