To repair a shower control valve, remove the shower handle and escutcheon plate to expose the valve, remove the old cartridge from the shower control valve, install a new cartridge and o-rings, and reinstall the shower handle and escutcheon plate. Shower valve rebuild kits are available at home improvement or plumbing supply stores. The tools required are a pencil, a screwdriver and a pair of pliers.
Before beginning the repair, turn off the water supply to the shower, block the drain to prevent small parts from falling into the opening, and spread a blanket in the tub or shower to protect the finish from dropped tools.
To install a new cartridge, remove the shower handle and escutcheon plate using a screwdriver. Loosen the locking nut with a pair of pliers and pull out the old cartridge. Push the new rubber seals and spring from the kit into the valve using a pencil. Insert the new cartridge into the valve, install an o-ring on the cartridge and tighten the locknut with the pliers. Finally, reinstall the escutcheon plate and shower handle with the screwdriver.
Turn the water supply back on and check for leaks. Additionally, ensure the water temperature does not exceed 120 degrees Fahrenheit to prevent scalding when taking a shower or bath.