Thermostat shows E11 and indicates an outdoor temperature sensor fault; outdoor readings are blank or frozen.

Safety first: Turn off power to the HVAC system at the breaker before handling sensor wiring.

• Step 1 – Locate outdoor sensor: Find the Geo‑compatible outdoor sensor mounted near the outdoor unit or on an exterior wall, away from direct sun.
• Step 2 – Inspect physically: Check the sensor housing for cracks, water ingress, or physical damage. Ensure the cable is not cut or chewed.
• Step 3 – Check connections: At the thermostat wall plate and at the sensor, verify that the sensor wires are secure in their terminals.
• Step 4 – Measure resistance: With the sensor disconnected and using a multimeter, measure resistance across the sensor leads. Compare to Geo's specified resistance vs. temperature chart (from the manual). A reading far outside expected range indicates a bad sensor.
• Step 5 – Replace sensor: If the sensor is out of spec or physically damaged, replace it with a Geo‑approved outdoor temperature sensor.
• Step 6 – Clear error: After replacement, power the system back on and allow several minutes for the thermostat to recognize the new sensor and clear E11.