Thermostat displays E3 and may not control the system correctly

Safety first: Turn off power to both indoor and outdoor HVAC units at the breakers before touching any wiring.

Steps to troubleshoot:

• 1. Confirm your system uses an outdoor sensor: Many Emerson smart thermostats (e.g., Sensi) do not require an external sensor. Check your installation manual or wiring diagram.
• 2. Inspect outdoor sensor: Locate the outdoor temperature sensor (often a small probe or capsule near the outdoor condenser). Check for physical damage, cuts in the cable, or corrosion at the connection points.
• 3. Check wiring connections: At the furnace/air handler control board or equipment interface module, verify the outdoor sensor wires are firmly landed on the correct sensor terminals and not shorted together.
• 4. Test continuity (advanced): With power off, use a multimeter to measure resistance across the sensor leads. Compare to the resistance/temperature chart in the Emerson documentation. Infinite resistance indicates open; near 0 ohms indicates short.
• 5. Replace sensor if faulty: If readings are out of spec, replace the outdoor temperature sensor with a compatible Emerson-rated sensor.
• 6. If sensor and wiring are good: The thermostat or interface module sensor input may be defective and require replacement.

When to call a professional: If you are not comfortable working at the outdoor unit or using a multimeter, contact an HVAC technician to test and replace the sensor.