getting an error that says 'fill water tank' but the tank is definitely full

Diagnosis

The Jura E8 utilizes a magnetic float system to detect water levels. Inside the water tank, a small magnetic ring floats on the surface of the water. As the water level drops, this magnet moves down; when it reaches a specific threshold, a reed switch (a magnetic sensor) located in the machine's chassis triggers the "Fill Water Tank" alert. When the machine displays this error despite a full tank, it means the circuit is not being closed, signaling to the control board that the water is missing.

This failure typically occurs for one of two reasons: mechanical obstruction or mineral buildup. Limescale (calcium carbonate) can accumulate around the float inside the tank, creating a "crust" that physically pins the float to the bottom or side of the tank, preventing it from rising with the water. Alternatively, the magnetic sensor in the machine's base may be obstructed by leaked coffee grounds or dried water residue, or the float itself has become detached from its magnetic housing, rendering it unable to trigger the sensor regardless of the water level.

How to Fix It

To resolve this, you must ensure the float is mobile and the sensor path is clear. Follow these exact steps:

1. Power down the machine and unplug it from the wall outlet to prevent any electrical shorts during cleaning.
2. Remove the water tank entirely and set it on a flat surface.
3. Inspect the float—the small, circular plastic piece that sits inside the tank. Gently shake the tank or use a clean chopstick to nudge the float. It should slide up and down the vertical guide rail with zero resistance. If it feels "gritty" or stuck, fill the tank with warm water and a splash of white vinegar, let it sit for 30 minutes, and manually move the float up and down to dissolve the scale.
4. Examine the bottom of the water tank and the base of the machine's tank cavity. Locate the small metal contact points and the recessed area where the tank seats. Use a microfiber cloth dampened with distilled water to wipe away any slime, coffee residue, or mineral deposits from these surfaces.
5. Check the "float guide" (the plastic channel the float moves in). If there is a visible air bubble trapped under the float, tilt the tank 45 degrees to release the air lock.
6. Reinsert the tank firmly, ensuring it clicks fully into the base. If the tank is slightly skewed, the magnet will not align with the internal reed switch, triggering the error.

If That Didn't Work

• Deep Descale the Tank: If the float is still sluggish, the scale may be internal. Fill the tank with a dedicated Jura descaling solution. Instead of running the machine's cycle, let the solution soak in the tank for 4 hours. This breaks down the calcium bonds holding the float in the "down" position.
• Test with a Spare Tank: If you have access to another E8 water tank, swap them. If the error disappears, the magnetic ring inside your original tank has lost its magnetism or the float is cracked and taking on water, causing it to sink.
• Inspect for Leaks: Check the area around the water tank base for any pooling water. If water has leaked into the machine's internal chassis, it can cause a short in the reed switch wiring, tricking the machine into thinking the tank is empty. Use a vacuum or dry cloth to clear any moisture from the tank cavity.
• Reset the Electronics: Perform a hard reset by unplugging the machine for 10 full minutes. This clears the temporary memory of the control board and forces it to re-poll the sensor status upon startup.

Heads Up

• Avoid Abrasives: Never use steel wool or harsh scrubbing pads on the tank's plastic or the machine's contact pins, as scratches can trap minerals and lead to more frequent sensor failures.
• Water Quality: Using filtered water (like a BWT or Brita filter) significantly reduces the likelihood of the float getting stuck due to limescale buildup.
• Tank Seating: Always ensure the tank is pushed all the way back. A gap of even 2-3 millimeters can be enough to move the magnet out of range of the sensor.
• Float Maintenance: Periodically check that the float is not floating "sideways" or wedged against the wall of the tank; it must move vertically to trigger the sensor.