Thermostat display shows E0 and will not communicate with the Ruud EcoNet / Ruud smart HVAC equipment

Safety first: Turn OFF power to the indoor unit (air handler/furnace) and outdoor unit at the breakers before touching any wiring. Low-voltage circuits can still short and damage boards.

• Verify ABCD wiring: Remove the thermostat from the wall plate. Confirm that the ABCD terminals and wire colors match exactly at the thermostat and at the Ruud indoor unit control board (A→A, B→B, C→C, D→D). Tighten any loose screws.
• Inspect cable: Follow the thermostat cable as far as accessible. Look for staples through the cable, crushed sections, or splices. Repair or replace damaged cable with 18/5 thermostat wire rated for low-voltage HVAC.
• Check low-voltage fuse: At the Ruud furnace/air handler control board, locate the 3–5A blade fuse. With power OFF, remove and visually inspect or test with a multimeter. Replace if open, but only after correcting any shorts.
• Isolate short: With power OFF, disconnect the thermostat wires at the indoor unit ABCD terminals. Restore power and see if the unit runs without the thermostat. If the fuse now holds, the short is in the thermostat cable or thermostat. If it still blows, the short is on the control board or other low-voltage wiring.
• Test thermostat: If wiring and fuse are good, temporarily connect a short known-good 18/5 cable between the thermostat and the indoor unit. If E0 clears, the original cable is faulty. If E0 persists, the thermostat main PCB or the Ruud control board communication circuit may be defective.
• When to call a pro: If you suspect a failed control board or thermostat communication chip, contact a Ruud-authorized technician for board-level diagnostics and replacement.