The cartridge is located inside the faucet handle. It is fitted to the shower body valve, which is inside the shower wall and fitted to the hot and cold water pipes. Both of the pipes feed into the shower body valve, which has two sides, one for each water pipe. The cartridge slides into the valve and serves as the main valve to mix the incoming water and produce a specific water temperature and flow rate. It does this using a simple valve system.
The faucet cartridge is made from two round discs flat on both ends. The discs are made of ceramic and machined to be perfectly flat. Therefore, when the two flat ends are fitted together the cartridge is water tight. The top cartridge rotates over the bottom one, which is fixed in place. The entire cartridge, including the two discs and the valves in the discs, is sealed. It is essentially one unit and when it breaks the entire cartridge is replaced.
A cartridge functions by allowing water to pass from the shower body valve into the spout or showerhead. It is important to note that it does not control where the water goes, a diverter in the spout does that. However, it will control flow rates and temperature. The only time water comes through the cartridge is when the top disc is rotated, and two inlet holes inside the discs align. This allows water through the cartridge into the spout. Turning the cartridge one way lets hot water through, because the hot water inlets are aligned and open, turning it the other way lets cold water through, because the cold water inlets are aligned.
Cartridges are the main culprit when leaks occur. A scratch on the surface of the ceramic disc or damage to the part causes water to drip through the spout. The only method of repair is a complete replacement of the entire cartridge. The cartridge is removed by taking off the handle and unscrewing the bonnet nut under the handle. It is then slid out of the faucet and a new one is inserted in its place. This is convenient and easy to accomplish and it only takes about 20 minutes.