the 12oz button on my k-duo plus only fills the cup halfway, no matter what cup size i select.

Diagnosis

The Keurig K-Duo Plus utilizes a dual-brew system that switches logic based on whether the machine thinks you are brewing a single cup or a full carafe. The root cause of a "half-filled" 12oz cup is almost always a mechanical failure of the carafe detection sensor. This sensor is a small, spring-loaded plastic lever located on the right side of the brewing chamber. When this lever is pushed up (or stuck up), the machine enters "Carafe Mode." In Carafe Mode, the internal software overrides the single-cup volume settings to prevent the machine from overflowing a small mug while it thinks a large glass carafe is underneath. Because the machine believes it is in carafe mode, it restricts the water flow and terminates the brew cycle prematurely to protect the heating element and prevent spills. You can confirm this by checking the display; if the "Carafe" indicator is lit or if the machine behaves as if it's preparing a pot even when you press the 12oz button, the sensor is the culprit. This is often caused by coffee grounds, crystallized mineral deposits from hard water, or a physical misalignment of the lever.

How to Fix It

To reset the sensor and restore full volume to your single-cup brews, follow these precise steps:

1. Power down the machine and unplug it from the wall outlet to ensure no electrical shorts occur during cleaning.
2. Remove the glass carafe and the carafe lid entirely to clear your field of vision.
3. Locate the small black plastic lever on the right-hand side of the brew station. This is the detection arm that the carafe pushes up when seated.
4. Using a cotton swab dipped in white vinegar or warm soapy water, clean around the base of the lever. Remove any dried coffee oils or calcium buildup that may be acting as a "glue," keeping the lever in the upright position.
5. Manually press the lever down with your finger and release it quickly. Repeat this 5 to 10 times to ensure the internal spring is fully engaging and the lever snaps back to its resting (down) position instantly.
6. Inspect the lever for any physical bends or cracks in the plastic. If the lever is stuck, gently wiggle it side-to-side to clear any debris trapped in the housing.
7. Plug the machine back in and attempt a 12oz brew without the carafe in place. The cup should now fill to the designated line.

If That Didn't Work

• Perform a Hard Factory Reset. Unplug the unit for at least 60 seconds. While unplugged, hold down the "Strong" and "Over Ice" buttons simultaneously for 10 seconds to clear the volatile memory. Plug the unit back in and test the 12oz cycle.
• Check for an Air Lock in the water line. If the pump is struggling, it may time out the brew cycle early. Remove the water reservoir, fill it with warm water, and firmly press down on the reservoir to force air bubbles out of the intake valve.
• Execute a Deep Descale. If you have hard water, calcium carbonate can build up inside the needles and the flow meter. Use a commercial Keurig descaling solution or a 50/50 mix of white vinegar and water. Run the largest brew cycle possible without a K-cup to flush the internal lines.
• Inspect the Exit Needle. Use a straightened paperclip to gently poke into the brew needle at the top of the chamber. If a piece of a K-cup foil seal is stuck in the needle, it creates back-pressure that can trick the machine's flow sensor into ending the brew early.

Heads Up

• Avoid using "extra-large" non-Keurig mugs that might lean against the side of the machine; if the mug pushes the sensor lever up, the machine will switch to Carafe Mode and shorten your drink.
• Always use filtered water to prevent the mineral buildup that causes these sensors to stick in the first place.
• Never force the carafe into the station; if it doesn't slide in easily, check for debris at the bottom of the brew head.
• Clean the brew needle weekly with a damp brush to prevent coffee oils from clogging the flow path.