In theory, replacing a shower faucet cartridge is an easy task. You turn the water off, dissemble the faucet handle to expose the cartridge and then pull the cartridge out. In reality, you will find more often than not that the cartridge is stuck, and despite your best efforts will not budge. Dirt, hard water deposits, or both can cause a cartridge to become stuck inside the valve. When this happens, the best and easiest way to get a stuck plastic or brass sleeve cartridge out of a shower valve is to use a cartridge puller.
Line up the cartridge puller, open side down, with the exposed stem of the stuck cartridge. Push down to seat the cartridge puller.
Turn the knob on the cartridge puller handle clockwise to secure, but do not over tighten.
Rotate the cartridge puller to the left and right while simultaneously pulling up to loosen and remove the stuck cartridge.
Press the spring pin button on the cartridge puller and slide the puller inside the brass sleeve opening.
Gently rotate the cartridge puller to the left and right until it lines up with the grooves inside the brass sleeve.
Turn the cartridge puller clockwise to loosen the brass sleeve and then pull up to remove the cartridge from the shower valve.
Inspect the cartridge to make sure you removed the retaining clip. If you need to take it out use a flathead screwdriver to pry it loose and then lift up to remove. Before purchasing a new cartridge, check the warranty on your shower faucet as some manufacturers offer a lifetime warranty on the shower valve and will provide a free replacement cartridge. If you no longer have the owner's manual for your faucet, call the customer service department of the manufacturer. Apply a dab of plumber's grease to the replacement cartridge before reinserting to make removal easier next time. After replacing the stuck cartridge, reassemble the faucet exactly as you took it apart and then turn the water on.
Before beginning any work, turn off the water supply to the shower faucet. If the turn-off valve for the shower faucet is behind the wall, turn the main water supply off at the point it enters your home.