Unless I missed it, you don't mention how old your unit is. If it's still under warranty, I'd get it to the dealer pronto
for warranty repair. They get really busy this time of year. If it's out of warranty and you've determined the problem is a faulty valve, you can replace the valve yourself. Provided there's enough slack in the affected water line, go to Home Depot and purchase a "Shark-Bite" brand 1/2" to 1/2" in-line shut-off valve. Cut the line just below and above the valve fittings (make sure the water pump is turned off
and your cuts are clean and square). Then just push the new valve onto one end of the cut water line, and push the other end into the new valve. (You need to push each line into the valve about 1".) The new valve is 1/4-turn. It is off when its handle is perpendicular to the water line; on when it is parallel to the line. If the water line doesn't have enough slack to connect to the new valve, you can purchase a short piece of PEX and additional fittings to piece in what you need. These things work like magic! I use them all the time when I work on manufactured homes, which also use PEX tubing. They are about double the price of similar PEX fittings, but you don't need that many. They are a lot more durable than PEX fittings, and you can install them easily without any bands and expensive crimping tools.
Hope you are able to track down the problem and get it taken care of.