I have the same setup on my 2013 Ranger 521 with 250 SHO. I have not been able to locate a place to purchase the sending unit. I did clear some debris in the water line going from the engine to the sending unit, but that still didn't allow me to register anything on the gauge once reattached. If you find a place to order the sending unit, please post in this thread. The advice above is a good way to troubleshoot and ensure the gauge is not faulty.