Safety: Disconnect power before servicing. Avoid touching live circuits.

Steps:

Locate humidity sensor: On Tosot units, the humidity sensor is often behind the front grille near the air intake, sometimes on a small daughter board or as a plastic module with vent holes.
Visual inspection: Check for dust buildup, moisture damage, or corrosion on the sensor and its connector. Clean gently with compressed air; do not use water or harsh cleaners.
Check connections: Ensure the sensor plug is fully seated on the control board or sub-board. Unplug and replug to improve contact.
Functional test: If the unit allows, compare displayed humidity to a separate hygrometer. If readings are wildly inaccurate or E3 persists, the sensor is likely bad.
Replace sensor: Install a new humidity sensor assembly specified for your Tosot model. Transfer any mounting clips or brackets.
Control board: If a new sensor does not resolve E3, the main PCB may not be reading the sensor correctly and may need replacement.

Dehumidifier shows E3 and shuts down or will not start dehumidifying.

Possible Causes