Thermostat displays E4 and may lock out heating or cooling

Safety: Turn off HVAC power at the breaker before working on low-voltage wiring.

• Remove thermostat: Pull the Lux thermostat off the wall plate and locate the remote sensor terminals (S1/S2 or similar).
• Disconnect sensor: Remove the remote sensor wires from the terminals. Restore power briefly to see if E4 clears with the sensor disconnected. If it clears, the fault is in the sensor or cable.
• Inspect sensor head: Locate the remote sensor module on the wall or duct. Check for moisture, corrosion, or physical damage.
• Check cable: With power off again, use a multimeter to measure resistance across the two sensor wires at the thermostat end. A reading near 0 ohms indicates a short in the cable or sensor.
• Isolate the short: Disconnect the sensor at its far end and re-test the cable alone. If the cable now reads open, the sensor itself is shorted and must be replaced. If the cable still reads short, replace the cable run.
• Install new sensor: Fit a new Lux-compatible remote sensor and reconnect to the thermostat, observing polarity if specified.