Thermostat displays E4 and outdoor temperature reading is incorrect or missing

Safety first: Shut off power to the HVAC system at the breaker panel before working on any thermostat or sensor wiring.

Steps to troubleshoot:

• 1. Inspect outdoor sensor location: Find the Resideo outdoor sensor and check for water entry, insect nests, or physical damage. Ensure it is not submerged or directly sprayed by sprinklers.
• 2. Disconnect sensor: At the thermostat or equipment interface module, disconnect the two outdoor sensor wires from the S1/S2 or OT terminals.
• 3. Measure resistance: With a multimeter, measure resistance across the two sensor leads. A reading near 0 ohms indicates a shorted sensor. Replace the sensor if shorted.
• 4. Check cable for shorts: If the sensor itself measures correctly, check the cable by measuring resistance between the two conductors with the sensor disconnected at both ends. A low resistance indicates a short in the cable.
• 5. Inspect for pinched points: Look where the cable passes through siding, conduit, or near the outdoor unit for crushed or cut insulation.
• 6. Replace components: Replace the outdoor sensor or run new cable as needed. Reconnect to the correct terminals and restore power.

When to call a professional: If E4 remains after sensor and cable replacement, the thermostat's sensor input may be damaged and should be diagnosed by a technician.