Thermostat displays E3 and shows incorrect or blank indoor temperature reading

Safety: Turn OFF power to the HVAC system at the breaker to avoid shorting low-voltage circuits when removing the thermostat.

• Check for environmental issues: Ensure the thermostat is not exposed to direct sunlight, drafts from supply vents, or heat sources that could cause abnormal readings. While this won't cause E3 directly, it can compound sensor issues.
• Remove thermostat: Gently pull the Ruud smart thermostat off its wall plate. Inspect the back for signs of corrosion, moisture, or burnt components.
• Clean contacts: Use a dry, soft brush or compressed air to remove dust from the sensor area and terminal contacts. Do not use liquid cleaners.
• Power cycle: With the thermostat removed, turn power OFF to the system for 2 minutes, then back ON. Reinstall the thermostat and see if the E3 clears after reboot.
• Check wiring stress: Ensure the thermostat wires are not pulling or twisting the thermostat body, which can stress the PCB and sensor. Provide slack and re-seat the thermostat firmly.
• Replacement: If E3 persists and the thermostat shows obviously incorrect temperatures (e.g., 0°F or 120°F indoors), the internal thermistor or PCB is likely failed. Replace the thermostat with a compatible Ruud EcoNet smart thermostat.