Thermostat displays E3 and may not control system correctly

Safety: Turn off power to the HVAC system at the breaker before touching any wiring.

• Confirm outdoor sensor is installed: Not all Honeywell Home smart thermostats use an outdoor sensor. If none is installed, verify installer setup and disable outdoor sensor in advanced settings if applicable.
• Inspect thermostat terminals: Remove the thermostat from the wall plate. Check the designated outdoor sensor terminals (often labeled S1/S2 or OT/OT). Ensure wires are firmly clamped and not broken.
• Check cable continuity: At the outdoor sensor location, inspect the Honeywell C7089 or similar sensor. Make sure the cable jacket is intact and not cut or chewed. Tighten the sensor terminal screws.
• Test sensor resistance: With power off, disconnect the two sensor wires at the thermostat and measure resistance across them with a multimeter. Compare to Honeywell’s temperature/resistance chart. Infinite resistance indicates an open sensor or broken wire.
• Repair or replace: If the cable is damaged, run a new 18/2 thermostat cable. If the sensor is open, replace the outdoor temperature sensor with a compatible Honeywell model.