A clogged mixing valve can create uneven, and possibly dangerous temperatures with the water that comes from your tap. If you suspect that the valve is clogged there are a few things you can do to correct the problem.
Step 1 - Clean around the Valve
Cleaning around the valve itself isn't going to get rid of a clog inside the valve, but it can help clear away any clogging that is preventing the valve from turning. With an automatic valve this can be especially problematic. To do this you want to turn off the heaters so they won't burn you in case the water leaks. Use a combination of baking soda and vinegar to gently remove any crust or build up around the valve.
Step 2 - Flush the System
If the clog is internal, it was likely caused by something in the system. The best way to deal with this is to flush the system. To do this you first want to drain the system through the drain in the floor. Make sure the shut off valve is closed while you drain, then open the valve back up while you flush the system. All systems are set up a little different, so refer to the manufacturers directions for full flushing advice.