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If the hot water will not turn off in your shower faucet, the cartridge that controls the water temperature is broken. This cartridge has two inlets, one for hot and one for cold. Either the hot water valve seal has cracked and will not seal out the hot water when you turn the handle towards the cold side, or the cold water inlet is completely blocked and will not let cold water in at all. The problem is an easy one to fix and requires no special skills.
Turn off the water supply to the faucet by closing the home water line near your water heater.
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Take off the handle on the shower faucet. Unscrew the set screw at the base of the handle with an Allen wrench, and then pull the handle off the faucet.
Take off the cartridge cover or bonnet, depending on which part your faucet uses, by loosening them with your hands or a large adjustable wrench. Remove the retaining nut or cartridge screws, depending on the type of faucet, on the cartridge with a screwdriver or wrench. Turn each one left to loosen and remove.
Pull out the cartridge by lifting it from the base of the faucet and replace it with a new one made for your specific type of faucet.
Replace the cartridge screws or retaining nut and bonnet or dome cap to rebuild the faucet. Install the handle and attach it by replacing and tightening the set screw with the Allen wrench.