Thermostat displays E10 and system does not respond to temperature changes

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breakers before inspecting wiring or replacing the thermostat.

Steps to troubleshoot:

• 1. Observe behavior: Note whether the system stays on constantly, never turns on, or ignores setpoint changes. This helps identify which relay (heat, cool, fan) may be affected.
• 2. Inspect thermostat wiring: Remove the thermostat and check that Y, W, G, and O/B wires are correctly landed and not shorted together.
• 3. Test system without thermostat (advanced): With power off, remove the thermostat and temporarily jumper R to Y (cool), R to W (heat), or R to G (fan) at the sub-base or furnace board. Restore power briefly to see if the equipment responds. If it does, the thermostat relay is suspect.
• 4. Remove any external relays or boosters: If an external relay panel or isolation relay is used, inspect it for burn marks or loose connections.
• 5. Replace thermostat if relay is faulty: Internal relays are not serviceable; replace the Emerson thermostat with a compatible model.

When to call a professional: If you are not comfortable jumpering low-voltage circuits or if the system does not respond even with jumpers, contact an HVAC technician to diagnose the control board and wiring.