Thermostat displays E2 and shows no or incorrect temperature reading

Safety first: Turn off power to the HVAC system at the breaker before working on thermostat wiring or outdoor sensor wiring.

Steps to diagnose and correct:

• Verify sensor presence: Confirm your specific Sensi model actually supports an outdoor or remote sensor. If the thermostat is configured for a sensor that is not installed, correct the configuration in the app or installer menu.
• Inspect wall plate terminals: Remove the thermostat from the wall plate. Check the designated sensor terminals (often labeled S1/S2 or similar). Ensure wires are fully inserted and terminal screws are snug, with no copper exposed or touching adjacent terminals.
• Check cable continuity: At the thermostat, gently tug each sensor wire to ensure it is not broken inside the wall. If accessible, inspect the cable run to the outdoor sensor for cuts, staples through the cable, or rodent damage.
• Test the sensor (if you have a multimeter): Disconnect the sensor wires at the thermostat. Measure resistance across the two sensor leads. Compare to the sensor's temperature/resistance chart from Sensi documentation. A reading of 0 Ω (short) or infinite (open) indicates a failed sensor or broken wire.
• Reset and reconfigure: After correcting wiring or replacing the sensor, power cycle the HVAC breaker and thermostat. Re-enter any installer settings related to outdoor sensor use.

When to replace: If wiring is intact and the E2 persists with a known-good sensor, the sensor input circuit on the thermostat main board is likely damaged and the thermostat should be replaced.