E04 appears and the thermostat shows an abnormally high or low temperature reading; heating may shut down for safety.

Safety first: Isolate power to the thermostat and heating system before working on wiring.

• Inspect sensor cable: At the thermostat base, disconnect the external sensor wires and visually inspect for crushed insulation or signs of moisture.
• Measure resistance: With a multimeter, measure resistance across the sensor. A reading near 0Ω indicates a short. If shorted, the sensor must be replaced.
• Dry out junctions: If the sensor is in a wet area (bathroom floor), check any accessible junction box for water ingress. Dry thoroughly and reseal.
• Check configuration: In the installation menu, confirm the correct sensor type (e.g. 10k NTC). If the wrong type is selected, the thermostat may interpret normal resistance as out-of-range.

If the sensor is embedded in the floor and shorted, replacement usually requires professional intervention. As a temporary workaround, disable the floor sensor and use room temperature control if the thermostat supports it.