There isn't much to the Posi-Temp valve.
There is three different kinds, does your have stops valves built in on the hot and cold?
If yes you may want to check to see if the stop valve is opened all the way or is not partially blocked.
If not than the only other thing it can be is the cartridge, which you should be able to get a new one under warranty from Moen or the store you bought it. The model number on the cartridge is 1222.