my black and decker cm1160b coffee maker just flashes 'add water' but the reservoir is full, it won't brew

Diagnosis

The "Add Water" flashing alert on the Black+Decker CM1160B is triggered by a failure in the water level detection system, specifically the magnetic float sensor. This model uses a small, buoyant plastic float containing a magnet that rises and falls with the water level. When the float is at the top, it triggers a reed switch in the machine's chassis, signaling that the reservoir is full. If the float becomes stuck due to mineral buildup (limescale) or trapped air bubbles, the sensor remains in the "low" position even when the tank is brimming. This issue is most common in regions with hard water, where calcium carbonate deposits create a sticky residue on the walls of the float chamber. This friction prevents the float from gliding upward. In some cases, a small piece of debris or a coffee ground fragment may have entered the reservoir and lodged itself beneath the float, physically blocking its ascent. You can confirm this by looking at the float through the transparent plastic of the reservoir; if the water is high but the float is sitting at the bottom or midway, the sensor is mechanically jammed.

How to Fix It

To resolve the stuck float and clear the "Add Water" error, follow these precise steps:

1. Disconnect the coffee maker from the wall outlet to prevent any electrical shorts during the cleaning process.
2. Lift the water reservoir straight up and out of the machine.
3. Locate the small, vertical cylindrical chamber at the rear of the reservoir. Inside this tube, you will see the white plastic float.
4. Using a thin, non-metallic object (such as a plastic coffee stirrer or a toothpick), gently reach into the chamber and manually push the float up and down several times. You should feel a slight resistance, but the float should move freely.
5. Fill the reservoir with a mixture of 50% white distilled vinegar and 50% warm water. Let this sit for 30 minutes to dissolve any calcium deposits clinging to the float and the inner walls of the sensor tube.
6. Pour out the solution and rinse the reservoir thoroughly under a high-pressure faucet stream, ensuring water is flushed directly into the float chamber to clear any loosened debris.
7. Dry the exterior of the reservoir completely, especially the area where it seats into the machine, to ensure a tight seal.
8. Reinsert the reservoir firmly and plug the machine back in. The "Add Water" message should now be replaced by the ready indicator.

If That Didn't Work

• Perform a deep-clean descaling cycle. If the float is free but the error persists, there may be mineral buildup on the internal reed switch inside the machine's body. Fill the reservoir with a commercial descaling solution or a concentrated citric acid mix and run a full brew cycle without a filter or coffee. This flushes the internal plumbing and can clear sensors that are partially obstructed by scale.
• Check for air locks in the reservoir seat. Sometimes a pocket of air prevents the reservoir from seating fully against the intake valve. Remove the reservoir and check the rubber gasket at the bottom for any cracks or debris. Wipe the gasket with a damp cloth and press the reservoir down firmly with a slight twisting motion to ensure it is fully engaged.
• Inspect the float for buoyancy failure. In rare cases, the float can develop a hairline crack, allowing water to seep inside. If the float is saturated, it will sink regardless of the water level. If you notice the float is not floating even in a standalone glass of water, the reservoir assembly must be replaced.
• Reset the control board by performing a hard power cycle. Unplug the unit for at least 10 full minutes. This allows the capacitors on the control board to discharge completely, which can clear a "ghost" error message that persists even after the mechanical float has been freed.

Heads Up

• Avoid using abrasive scrubbing pads inside the float chamber, as scratches in the plastic can create new snag points for the float to get stuck.
• Always use filtered water if you live in a hard-water area to prevent the calcium buildup that causes this specific sensor failure.
• Never use soap or detergents inside the water reservoir; any residual suds can create air bubbles around the float, causing it to "bounce" and trigger intermittent "Add Water" warnings during a brew.
• Clean the reservoir with a vinegar solution once every 30 brew cycles to maintain the smooth movement of the magnetic sensor.