E20 displayed and thermostat reports a sensor calibration error; measured temperature is offset significantly.

Safety first: No special risk for calibration; avoid opening mains-powered units with power on.

• Compare with reference: Place a reliable thermometer near the thermostat and compare readings.
• Reset calibration: In the menu, go to Settings > Temperature calibration and reset the offset to 0 or factory default.
• Re-enter small offset: If needed, apply a small correction (e.g. ±1–2°C) and verify that E20 clears.

If E20 persists and the reading is far from reality, the internal sensor is likely defective and the thermostat should be replaced.