Thermostat displays E1 and cannot communicate with the HVAC equipment

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breaker before touching any wiring.

• Step 1 – Power cycle: Turn HVAC breakers off for 2–3 minutes, then back on. Wait for the ecobee to reboot and check if E1 clears.
• Step 2 – Check furnace board fuse: Open the furnace/air handler panel. Locate the low-voltage 3A automotive-style fuse on the control board. If blown, replace with the same rating only. If it blows again, there is a short in the thermostat wiring.
• Step 3 – Inspect thermostat wiring: Remove the ecobee from its backplate. Verify R, C, W, Y, G (and O/B if used) are firmly clamped and no bare copper is touching adjacent terminals. At the furnace board, verify the same wires are landed on R, C, W, Y, G.
• Step 4 – Check PEK (if installed): Confirm the PEK wires are connected to the correct terminals on the furnace board and that the labeled wires at the thermostat match ecobee’s PEK instructions. Reseat all connectors.
• Step 5 – Test with a simple R–C power check: With power on, measure 24 VAC between R and C at the thermostat backplate using a multimeter. If no voltage is present, the issue is in the furnace board, transformer, or wiring.
• Step 6 – Replace thermostat if needed: If 24 VAC is present and wiring is correct but E1 persists, the ecobee thermostat’s communication circuit may be defective and the thermostat should be replaced.