The shower cartridge is the single part in your faucet that controls the water flow rates and temperature. It is also frequently replaced to upgrade the faucet performance and to fix leaks. If the cartridge is stuck, there are ways to extract it without calling a plumber or damaging the faucet.
All cartridges have a retaining mechanism holding them in place. Without it the cartridge will simply pop out due to the water pressure and force behind it. Remember, the cartridge prevents water from coming through the faucet when the handles are turned to the off position. Look around the stem of the cartridge, the part on the end, and around the base of the faucet body where the cartridge is installed for clips, or nuts and washers, and remove them with pliers or by hand. Then attempt to remove the cartridge.
Loosen Sediment Seals
A sediment seal makes removing any cartridge difficult. This is a ring of sediment build-up around the edge of the cartridge where it meets the faucet body. To break this seal, grip the end of the cartridge stem with a pair of light duty pliers. Gently turn the cartridge back and forth until the sediment seal is broken. Now, carefully slide the cartridge out of the faucet.
If the sediment seal is really bad a cartridge puller is the best bet. This is a tool used specifically to remove cartridges. It is round, has a threaded end and fits only your particular cartridge. Purchase one from your faucet manufacturer or a faucet parts store. Attach the end to the stem of the cartridge by screwing it onto the stem. Continue to turn it to break the sediment seal and pull the cartridge out by pulling back on the cartridge puller's handle.
If you have an older faucet where the handle screws onto the stem of the faucet, there is an alternative method to using a cartridge puller. After you remove all the retaining clips and nuts, reattach the handle by screwing it onto the end of the cartridge stem. Then, using the handle as a grip, pull back and remove the cartridge. Twist the handle back and forth to break the sediment seal if necessary.