Thermostat displays E2 and temperature reading is frozen or missing

Safety first: Switch off power to the furnace/air handler at the breaker before working on the thermostat wiring.

Steps to troubleshoot:

• 1. Inspect thermostat face: Remove the Resideo thermostat from the wall plate. Check the back of the thermostat for signs of moisture, corrosion, or damage around the sensor area.
• 2. Check remote sensor wiring: If a remote indoor sensor is used, disconnect its wires from the S1/S2 (or similar) terminals. Make sure the two sensor wires are not touching each other or any other terminals.
• 3. Measure resistance: With the sensor disconnected, measure resistance across the sensor leads. A reading near 0 ohms indicates a shorted sensor. Replace the sensor if shorted.
• 4. Inspect cable path: Look along the sensor cable route (attic, basement, wall penetrations) for pinched or damaged insulation that could cause a short.
• 5. Test without sensor: Leave the sensor wires disconnected and power the system back on. Many Resideo models will revert to the internal sensor. If E2 clears, the external sensor or its wiring is the issue.
• 6. Replace components: Replace the remote sensor or thermostat if the internal sensor is shorted and cannot be repaired.

When to call a professional: If you cannot locate the short or the thermostat still shows E2 with all external sensors removed, contact an HVAC professional or Resideo technical support.