E18 displayed and thermostat indicates a humidity sensor fault (on models with integrated humidity measurement).

Safety first: Disconnect power before opening the thermostat housing.

• Visual inspection: Open the front cover and locate the humidity sensor (often a small perforated or exposed chip). Check for dust, paint, or debris.
• Gentle cleaning: Use a dry, soft brush or compressed air to remove dust. Do not use water or solvents.
• Check location: Avoid installing the thermostat in very humid areas (showers, above kettles) where condensation can form. Relocate if necessary.

If E18 persists and humidity readings are clearly wrong or absent, the sensor is likely damaged. As it is usually integrated on the PCB, replacing the thermostat is the practical solution.