Thermostat displays E3 and will not control the system correctly

Safety: Turn off power to the HVAC system at the breaker.

• Locate sensor wiring: Remove the thermostat from the wall plate and identify the external sensor terminals (often labeled S1/S2 or RS+ / RS- on Lux models that support remote sensors).
• Check connections: Ensure the remote sensor wires are firmly secured under the terminal screws. Tighten gently if loose.
• Inspect cable: Follow the sensor cable as far as accessible. Look for cuts, staples through the cable, or crushed sections.
• Test sensor continuity: With power off, disconnect the two sensor wires at the thermostat and measure resistance across the pair using a multimeter. A typical Lux remote sensor thermistor will show a finite resistance (often 5k–20k ohms depending on temperature). Infinite resistance indicates an open sensor or broken wire.
• Bypass test: Temporarily connect a known-good Lux remote sensor directly at the thermostat terminals. If E3 clears, the original sensor or its cable is faulty.
• Replace as needed: Replace the remote sensor or run new low-voltage cable if the existing cable is damaged.