Wrong cartridge jammed in the valve

1. Stop and shut everything off

Keep the water off and open the handle to drain pressure. If you forced the part in, stop forcing it. More force mostly damages the brass valve body, and a damaged valve body means opening the wall.

2. Back it out the way it went in

• Pull straight out by the stem. Don't lever against the brass with a screwdriver. Delta specifically warns against prying.
• If it's a Moen-style single-handle cartridge and it won't come out, Moen's 104421 puller is made for this (1200, 1222, 1225). See stuck cartridge for the steps and the broken-stem methods.
• Check that no O-rings, grommets or bits of plastic stayed behind. Then clean and flush the valve body.

3. Figure out what you actually need

• Use the valve, not the trim. Retrofit trims and replaced handles are common. The valve body stamp (Pfister: 0X8, 0X9 and so on) and the old cartridge itself are better evidence.
• Use your model's parts diagram. Delta staff say to install the part shown on your model's diagram rather than a lookalike, even between two Delta cartridges that look almost the same.
• Some valves only take one type. Pfister, for example, says its pull-and-turn Avante valve needs the pull-and-turn cartridge. Converting to a turn-only cartridge means a different valve behind the wall.

Still unsure? Go to identify your shower valve brand, then the brand page.

4. When to call a plumber

Call one if the brass valve body is scored or cracked, threads are damaged, the valve turns in the wall, or water shows up inside the wall. Those aren't cartridge problems anymore.