Thermostat shows E04 and the room temperature reading is clearly wrong (e.g., shows 5°C when room is warm).

Safety first: Turn off power at the breaker before removing the thermostat from the wall.

Steps to diagnose:

• Compare with a reference thermometer: Place a reliable thermometer near the thermostat and compare readings. If the Mysa reading is off by more than a few degrees and E04 is present, the sensor is suspect.
• Check for drafts or heat sources: Ensure the thermostat is not directly above a baseboard heater, in direct sunlight, or in a draft that could skew readings.
• Inspect the sensor area: Remove the thermostat from the wall and look at the front PCB area where the internal NTC sensor is located (usually near vent holes). Check for dust buildup, corrosion, or physical damage.
• Clean gently: Use compressed air to remove dust from the sensor vents. Do not use liquids.

If E04 remains and the temperature reading is clearly inaccurate, the internal NTC sensor or its circuit has failed. Because the sensor is typically soldered to the main board, the practical repair is to replace the thermostat.