Thermostat displays E6 and cannot control the HVAC system

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breaker before working on low‑voltage wiring.

Steps to resolve:

• Verify wiring at thermostat: Remove the thermostat and inspect each wire. Confirm that each conductor is in the correct labeled terminal (R, C, Y, W, G, O/B, etc.) according to your system type and Sensi wiring guide. Tighten any loose terminal screws.
• Inspect wiring at furnace/air handler: With power off, remove the blower door and locate the control board. Confirm that thermostat wires are securely attached to the matching R, C, Y, W, G, and O/B terminals. Look for any bare copper touching adjacent terminals.
• Look for shorts or damage: Trace the thermostat cable where visible. Repair or replace any sections with crushed insulation, exposed copper, or signs of rodent damage.
• Reset the system: After correcting wiring, reinstall the furnace door (to engage the safety switch), restore power at the breaker, and allow the thermostat to reboot. Check if E6 clears.
• Test with a jumper (advanced): If comfortable and with power on, briefly jumper R to G at the furnace control board to see if the blower runs. If it does, the board is likely good and the issue is between the board and thermostat.

When to replace: If wiring is correct and intact and E6 persists, the thermostat's internal communication or sensing circuitry may be damaged. Replace the thermostat or consult an HVAC technician.