Diagnosis
This means the condensate pump (the little box on the back with the hose connection) is either air-locked or the hose has a high point that's creating too much back pressure for it to push the water through.
How to Fix It
First, check the hose path. It must run downhill from the pump outlet all the way to the drain. Any upward loop or high spot will trap water and air, stopping the flow. Straighten the hose so it has a constant downward slope. If that's good, unplug the unit, disconnect the hose from the pump outlet, and pour a cup of water directly into the pump's outlet hole to prime it and clear any air.
If That Didn't Work
The pump impeller might be jammed with scale or debris. You'll need to access the pump. Unplug everything, remove the few screws holding the pump housing to the back of the unit, and pull it off. Check the small impeller inside the outlet port—you can often clear it with a toothpick or by flushing with vinegar.
Heads Up
This pump isn't super powerful. It's designed to lift water about 10-15 feet vertically. If you're trying to pump it up a long rise to a sink, it might just not have the muscle. A gravity drain is always more reliable.