If you have warm water, the valve is not the problem, but please feel free to buy one just because you read it somewhere.
Common sense tells us that the hot side of the valve still works because there is a warm water mix. If it were a bad valve you would not get warm water, just cold only.
The little temperature switch on the control panel must send power to the valve to achieve hot, warm and cold temps. A technician would whip out his OHM meter and check the temp switch contacts using the handy diagram inside the control panel. It's all there.