A faucet cartridge is the replaceable valve body inside a single- or two-handle faucet that controls water flow and, in mixing faucets, the hot and cold blend. Most modern cartridges use ceramic discs or a plastic spool with rubber O-rings, and each brand and model takes a specific part. A worn cartridge causes drips and stiff handles.
Why it matters to you
A steady drip at the spout usually means a worn cartridge, and in a shower the same kind of part, often with a pressure-balancing feature, is what prevents a sudden scald when someone flushes a toilet.
Practical guidance
Close the fixture's supply stops before removing a handle, and bring the old cartridge to the parts counter to match it exactly. If a shower suddenly runs scalding or cold-only, stop using it until the mixing cartridge is checked.