No — it is nearly always the cartridge inside the valve, a part costing between fifteen and sixty dollars, and the valve body stays in the wall. The job is an hour once you have identified the right cartridge for your brand.
What a cartridge does and why it fails
Behind the handle is a cartridge that controls both mixing and flow. Its seals are rubber or ceramic and they wear, and hard water accelerates it. When they wear you get a handle with slop in it, a drip that will not stop however hard you turn it off, or water that runs hotter and colder on its own.
Turning the handle harder to stop the drip is the thing to avoid. It does not seat a worn seal, and it does snap handles and score the valve body — which turns a cheap job into an expensive one.
Identify the brand before you buy anything
Cartridges are not universal and there is no generic. Look for a maker's name on the handle, the escutcheon or the trim. If there is nothing visible, pull the handle and photograph the cartridge before going to the store — the counter staff can usually identify it on sight. Some brands offer a free lifetime cartridge if you call them, which is worth ten minutes on the phone.
The job
- Shut off the water. Some valves have integral stops behind the trim plate; otherwise it is the house main.
- Open the shower to drain the line.
- Plug the drain before you remove anything. A dropped retaining clip goes down an open drain immediately, and that clip is what stops the cartridge firing out under pressure.
- Remove handle, escutcheon and retaining clip, and pull the cartridge straight out. A puller tool is cheap and is worth it on a seized one.
- Grease the new cartridge with silicone plumber's grease, insert in the same orientation, reassemble.
- Turn the water back on slowly and check for weeping before the trim goes back.
Two things that catch people out
Orientation. Fit some cartridges 180 degrees out and hot and cold swap over. If that happens, it is not a plumbing fault, just a rotation.
The scald stop. Many valves have an anti-scald limiter that needs setting after a cartridge change. If your maximum temperature is now lukewarm, that ring is the reason, not the new part.
When it stops being a DIY job
If the valve body itself is leaking behind the wall, if the cartridge is seized so hard it will not draw, or if there is no access and the valve is not a serviceable type, you are into opening the wall. That is also the moment to think about whether the tile you would be cutting is tile you were planning to keep.