Machine displays 'Fill water tank' even though the Melitta water tank is full and correctly inserted.

Safety first: Unplug the coffee maker before inspecting the tank sensor area.

Steps:

• Clean the tank: Remove the water tank, empty it, and rinse thoroughly. Check that the internal float moves freely; soak in warm water with mild detergent if it is sticky.
• Check the float magnet: Verify that the magnet is present in the float (often a small cylinder). If missing or corroded, replace the tank.
• Clean sensor area: Wipe the area on the machine where the tank sits, especially any clear plastic window or sensor housing.
• Re-seat tank: Refill and insert the tank firmly until it clicks into place.

If the message persists, the internal water level sensor (reed switch or Hall sensor) may be faulty and require replacement.