the 'add water' light is on but the reservoir is completely full, it won't brew

Diagnosis

The Keurig K-Supreme Plus SMART utilizes a magnetic float system located at the base of the water reservoir to detect water levels. When the reservoir is full, a small buoyant magnet rises within a plastic chamber, triggering a reed switch in the machine's base. If the "Add Water" light remains illuminated despite a full tank, the float has likely become physically jammed due to friction or is trapped by calcium carbonate (scale) deposits that have solidified around the float's perimeter. In some cases, the issue is not mechanical but electrical. Mineral buildup can create an insulating layer on the sensor contacts or the magnetic field is being interfered with by debris. Because the machine's logic board relies on a binary "high/low" signal from this float, any obstruction that prevents the float from reaching the top of its travel path will cause the machine to believe it is empty, triggering a safety lockout that prevents the brew cycle from initiating to protect the heating element from dry-firing.

How to Fix It

1. Power down the machine by unplugging it from the wall outlet to reset the sensor logic.
2. Remove the water reservoir from the machine. Inspect the bottom of the tank for any visible debris or slime.
3. Locate the small, rectangular plastic chamber at the bottom of the reservoir. This is the float housing.
4. Hold the reservoir firmly and shake it vigorously in a downward motion. You should feel or hear a small plastic piece clicking or sliding up and down inside that chamber. If it doesn't move, tap the bottom of the reservoir firmly against the palm of your hand several times to break the seal of any mineral buildup.
5. Fill a bowl with warm water and a tablespoon of white distilled vinegar. Submerge only the bottom 2 inches of the reservoir in this solution for 15 minutes to dissolve calcium deposits around the float.
6. Rinse the reservoir thoroughly with fresh water and use a lint-free microfiber cloth to dry the exterior bottom and the sensor contact points on the machine's base.
7. Reinstall the reservoir, ensuring it seats firmly and completely into the base. Plug the machine back in and check if the "Add Water" light has extinguished.

If That Didn't Work

• Perform a Hard Factory Reset: Unplug the unit for at least 10 full minutes. While unplugged, hold the brew button for 15 seconds to discharge remaining capacitors. This clears the temporary memory of the sensor error.
• Deep Descale the Internal Lines: Even if the tank is full, internal scale can confuse the pressure sensors. Run a full descaling cycle using Keurig Descaling Solution or a 50/50 white vinegar and water mix. Run the cycle without a K-Cup inserted to ensure maximum flow through the needles.
• Inspect the Base Valve: Check the rubber gasket/valve at the bottom of the reservoir. If it is warped or has a piece of debris lodged in it, the water may not be flowing into the internal tank fast enough for the sensor to register the level correctly. Clean the valve with a damp cloth.
• Check for Air Locks: Lift the reservoir up and down rapidly 3-4 times while the machine is on. This can sometimes force air bubbles out of the sensor chamber, allowing the float to move freely again.

Heads Up

• Use Filtered Water: To prevent the float from sticking again, avoid using unfiltered tap water. High mineral content (hard water) is the primary cause of float failure in the K-Supreme Plus series.
• Avoid Harsh Chemicals: Never use bleach or abrasive scrubbers on the bottom of the reservoir; this can damage the plastic housing and cause the float to leak or jam permanently.
• Monthly Maintenance: Every 30 days, shake the reservoir while empty to ensure the float is moving freely. This prevents scale from "welding" the float in place over time.
• Water Level Warning: Do not overfill the reservoir beyond the "Max" line, as this can put undue pressure on the seal and potentially cause leaks into the electronics base.