Display shows 'Fill water tank' even though the tank is full.

Safety first: Unplug the machine before internal sensor inspection.

Step-by-step:

• 1. Clean tank: Empty and wash the water tank, ensuring the float moves freely. Remove scale around the float guide.
• 2. Check magnet: Verify that the float's magnet is present and not corroded.
• 3. Clean sensor window: Wipe the transparent window on the tank and the corresponding area on the machine.
• 4. Reseat tank: Refill and firmly reinsert the tank, ensuring it engages fully.
• 5. Professional service: If the issue remains, the internal level sensor (reed or Hall) may be defective and should be replaced by Siemens.