If your shower has stopped holding a steady temperature or the handle has become stiff, the valve behind the wall is usually the reason.
Scale on the valve
In hard water, mineral buildup collects in the mixing valve cartridge. It rarely fails suddenly — it gets gradually stiffer and less reliable over months until someone gets a shock when a tap runs elsewhere in the house. Replacing the cartridge restores proper control.
Blocked shower heads
A weak or scattered spray is nearly always scale in the head. Soaking it in vinegar clears most of it. If it comes back within months, that is your water telling you something, and treating it is the lasting fix.
You may not need to lose your tile
Worth knowing before you agree to anything. Many valve repairs can be reached through the existing trim plate, or through a small opening in the wall of the room behind — often a closet. Before accepting a quote that involves taking out tile, it is entirely reasonable to ask whether the valve can be reached from behind.
A stain on the ceiling below
Usually the shower base, the drain connection, or grout letting water past. Three different repairs, and knowing which before opening anything is the difference between a small job and losing the bathroom.