A clogged shower valve will cause the shower's water pressure to decrease. This drop in water pressure can happen suddenly or slowly over time, depending on the source of the clog. A buildup of hard water deposits involves a slow process, meaning you may not realize the water pressure has decreased until the clog becomes severe. Sediment from a recent water line break or other plumbing problem will cause a more sudden clog to occur in the shower's valve.
Close the house's main water valve to cut off the flow of water from the house, or close both valves in the access panel located on the wall behind the shower's faucet. Turn on the shower to make sure the water flow has been completely cut off.
Remove the screw holding the control handle on the faucet, located either at the bottom of the handle or under the cap in the middle of the handle. Pull the handle off the valve assembly and unscrew any sleeves or adapters from the assembly.
Twist the valve out of the pipe using an adjustable wrench. Place the valve in a dish full of warmed white vinegar so the valve is completely covered.
Rinse the valve with warm water and wipe it down with a clean paper towel. Twist the valve into place using the adjustable wrench. Screw back on any sleeves or adapter pieces.
Slide the control handle back on the valve assembly and replace the screw you removed before. Turn the water to the shower back on and check for leaks.