i keep getting a ‘prime’ message on the screen and it won’t brew, even though the tank is full.

Diagnosis

The "Prime" message on the Keurig K-Supreme Plus is a system alert indicating that the internal water pump is running dry or cannot create enough suction to move water from the reservoir to the brew head. This is almost always caused by an "air lock"—a pocket of air trapped within the internal tubing that prevents the pump from creating a vacuum. When the pump spins without water, it cannot build the necessary pressure to trigger the sensor that tells the machine it is ready to brew, resulting in a persistent "Prime" loop. This typically occurs after the machine has been stored for a long period, if the water reservoir was allowed to run completely empty during a brew cycle, or if the machine was tilted during transport. You can confirm this is an air lock if you hear the pump making a loud, strained vibrating noise for several seconds before the "Prime" message reappears, despite the water tank being physically full to the max fill line.

How to Fix It

To clear the air lock and prime the pump, you must force the machine to cycle water without the resistance of a K-Cup pod. Follow these exact steps:

1. Remove any K-Cup pod from the holder and ensure the pod chamber is completely empty.
2. Fill the water reservoir to the MAX line using filtered water. Ensure the reservoir is pushed down firmly until you feel it click into the base; if it is floating even 1mm too high, the intake valve won't engage.
3. Place a large mug or a 12oz beaker on the drip tray to catch the discharge.
4. Close the handle firmly.
5. Press the "8oz" button. The machine will attempt to brew. If it stops and displays "Prime" again, repeat this process.
6. If the pump is struggling, gently lift the water reservoir up and down rapidly 3-4 times while the pump is running. This "jiggles" the valve and often breaks the air bubble's surface tension.
7. Continue running 8oz cycles without a pod until a full stream of water flows consistently and the "Prime" message disappears from the LCD screen.

If That Didn't Work

• Check the Reservoir Valve: Inspect the rubber gasket at the bottom of the water tank. If there is a piece of debris or a mineral buildup on the valve seal, the pump will suck air instead of water. Clean the valve with a damp cloth and ensure the O-ring is not pinched or cracked.
• Perform a Hard Power Reset: Unplug the machine from the wall outlet for exactly 60 seconds. While unplugged, hold the power button for 5 seconds to discharge remaining capacitors. Plug it back in and attempt the priming sequence again. This resets the internal sensor logic.
• Clear the Exit Needle: Use a straightened paperclip to gently poke into the brew head needle (the one that punctures the top of the pod). If a piece of foil or coffee ground is blocking the exit, the pump will hit a "pressure wall" and trigger a Prime error because it thinks the line is blocked.
• Deep Descale: If you have hard water, calcium deposits can create micro-obstructions in the narrow internal tubing. Run a full descaling cycle using white vinegar or Keurig Descaling Solution, letting the solution sit in the tank for 30 minutes before attempting to brew.

Heads Up

• Avoid Running Dry: Never let the water reservoir drop below the minimum fill line during a brew; this is the primary cause of air locks in the K-Supreme Plus model.
• Filter Maintenance: If you use a charcoal water filter, replace it every 2 months. A clogged filter can restrict flow enough to trigger a "Prime" alert.
• Water Temperature: If the machine has been in a cold garage, the internal lines may be too viscous. Move it to room temperature for 2 hours before priming.
• Tank Seating: Always ensure the tank is seated flush. If you have added an aftermarket water filter or a different tank, it may not be triggering the internal float switch.