The shower faucet cartridge is one of the more important parts of your faucet because it operates and controls the hot and cold water. Without it, your shower faucet water would either be extremely hot or too cold, and you could not adjust it properly. This part does not last forever. Eventually it needs to be changed and, when it does, you must replace it with the exact same cartridge model. Identifying the cartridge properly is a big part of making this essential home repair.
Turn off the shower faucet water by closing the main water valve in the home. It is typically located near the hot water heater. You also can check behind the shower wall for a plumber's access panel and turn off the water valves behind this panel. This shuts off the water to the shower only.
Remove the shower cartridge by taking off the cover on the top of the handle and loosening the set screw with an Allen wrench. The set screw also may be located on the side of the handle. In this case, there will be no cover. Simply unscrew the screw with the Allen wrench. Slide off the handle and remove the bonnet cap by turning it to the left. If there is no bonnet, then your faucet uses a cartridge clip. Pry it up with a pair of pliers and pull it away from the cartridge head. Remove any other sleeves or washers securing the cartridge to the fixture head. Then slide the cartridge out.
Look on the front and back of the cartridge for an identification number. Some cartridges have one, many will not. Copy this down on a piece of paper and bring the part to a hardware store.
Ask the clerk to help you make a visual parts match using their faucet parts books. If the parts number is legible, offer this as assistance to help narrow down your search. If you know the make or model of the faucet, tell the clerk which model it is. This will be a big help in identifying the cartridge. In fact, most cartridge parts can be ordered with this information alone.