How do I repair a shower mixer trap?

A shower mixer trap is part of a larger valve and can be flushed out by opening the valve, shutting off water flow and then briefly restarting it in sequence. Removing the valve cap and installing a new one is a decent-sized project, requiring tools like a utility knife, a sieve or paper plate, pliers, a screwdriver and other basic plumbing tools.

The sieve or paper plate serves to catch debris. This is flushed from the trap when the valves are opened.

1. Uncover the cap
The first step is to open the valve cap. It can be exposed with pliers and a utility knife.
2. Flush and remove the valve
The next step is to shut off water flow and remove the cap. Then flush the trap by turning each flow valve on and then off.
3. Install the new valve
Once the valve has been flushed and removed, it can be replaced.