Triton 82300170

A: 

Yes it could be a faulty stepper motor or a faulty signal from the board.

 

If the stepper motor is working then you should be able to hear it running when you change temperature.

If it is not running then check that the motor is getting a power supply from the board.

 

Basically if it is getting a supply and not working then you probably have a faulty motor if it is not getting a supply then you probably have a faulty power board.

A: No need to turn water off but certainly isolate electrics.
Disconnect the leads to the motor the simply remove the 2 screws that hold it in place and it lifts off and replacement simply goes backin though you may have to twist it slightly to get splines to line up the turn to align screw holes and reconnect leads.