Tempstar communicating furnace thermostat or interface displays E0; system not responding to commands.

Safety: Turn off power to both furnace and thermostat (if powered separately) before working on wiring.

• Step 1 – Power cycle system: Turn off furnace breaker and thermostat power for 1 minute, then restore. Many communication errors clear after a full reboot.
• Step 2 – Inspect communication wiring: For Tempstar communicating systems, verify the ABCD (or similar) terminals are wired consistently from thermostat to furnace (A to A, B to B, etc.). Tighten all screws.
• Step 3 – Check polarity: Ensure R and C are not reversed and that any external transformer is wired correctly.
• Step 4 – Test with short cable: Temporarily connect the thermostat to the furnace with a short piece of new thermostat cable. If communication works, the in-wall cable is likely damaged and should be replaced.
• Step 5 – Replace faulty component: If wiring is correct and the error persists, either the Tempstar communicating thermostat or the furnace control board may be defective. Swap components as directed by Tempstar service literature.