Thermostat displays E3 and outdoor temperature is not available

Safety first: Turn off power to the HVAC equipment at the breaker before touching thermostat or sensor wiring.

Steps to troubleshoot:

• 1. Verify sensor presence: Confirm that a Resideo outdoor sensor (e.g., C7089U1006) is actually installed and wired to the thermostat or equipment interface module.
• 2. Inspect outdoor sensor: Locate the sensor near the outdoor unit or shaded exterior wall. Check for physical damage, UV-cracked housing, or chewed wires.
• 3. Check connections at thermostat: Remove the thermostat from the wall plate and verify the outdoor sensor wires are firmly attached to the correct S1/S2 or OT terminals.
• 4. Continuity test: With power off, disconnect the sensor wires at the thermostat and measure resistance across the two wires going to the sensor. An infinite reading indicates an open circuit in the sensor or cable.
• 5. Direct sensor test: If possible, measure resistance directly at the sensor terminals outdoors. Compare to the Resideo temperature/resistance chart. Replace the sensor if it reads open.
• 6. Replace or rewire: Replace the outdoor sensor if failed, or run new low-voltage cable if the existing cable is damaged.

When to call a professional: If the sensor and wiring test good but E3 persists, the thermostat or interface module may have a failed sensor input and should be evaluated by a professional.