What Does This Error Mean?

A Keurig not detecting the water reservoir is usually caused by the reservoir not being fully seated on the base, or mineral buildup on the magnetic water sensor. Remove the reservoir, wipe the magnet on the reservoir bottom and the sensor well on the base, then reseat firmly.

Affected Models

Keurig K-Classic
Keurig K-Elite
Keurig K-Select
Keurig K-Slim
Keurig K-Supreme
Keurig K-Café

Common Causes

Reservoir not fully seated — the magnet is not aligned with the base sensor
Mineral scale or residue on the reservoir bottom magnet or the base sensor well
Small air pocket preventing the reservoir from sitting flush on the base
Internal water level sensor clogged with scale buildup
Software state requiring a power cycle reset

How to Fix It

Remove and fully reseat the water reservoir.

Lift the reservoir straight up and off the Keurig base. Inspect the bottom of the reservoir for the small circular magnet. Place the reservoir back onto the base and press down firmly until it sits flush — there is no click, but it should not rock.
Clean the reservoir magnet and base sensor.

Wipe the bottom of the water reservoir with a damp cloth, paying attention to the magnet area. Also wipe the sensor area inside the base well where the reservoir bottom sits. Mineral scale or coffee residue on the magnet is the most common cause of this detection error.
Power cycle the Keurig.

Unplug the Keurig from the wall outlet for 30 seconds, then plug it back in. This resets the internal water detection routine. Reseat the reservoir again after the power cycle, then turn the machine on.
Fill the reservoir above the minimum fill line.

The Keurig sensor requires the water level to be above the minimum line marked on the reservoir side. Even if the reservoir looks full from above, water just at or below the minimum line can cause a false Add Water reading. Fill to at least the halfway mark.
Descale the Keurig.

Scale buildup inside the water path can restrict flow past the internal sensor and cause false empty readings. Run a full descale using Keurig's descaling solution following the instructions in Settings > Maintenance > Descale. Descaling often resolves persistent water detection errors.

When to Call a Professional

If thorough cleaning and reseating fail and the Keurig consistently reports Add Water with a full reservoir, the magnetic water level sensor may need replacement. Contact Keurig customer support.