How do I know what Moen shower cartridge to buy?

To know what Moen shower cartridge to buy, you must correctly identify your existing valve and cartridge because Moen cartridges are not interchangeable. The easiest way is to remove the old cartridge and match it visually. Turn off water to the shower, remove the handle and escutcheon plate, pull the retainer clip, and pull out the cartridge. Compare it to known types. If your shower handle pulls out to turn on and then rotates for temperature, and the cartridge is about 4 inches long with a white plastic body and brass stem with two small alignment tabs, you need the Moen 1222 Posi-Temp cartridge, which is the most common since the 1980s. If your handle does not pull but just rotates 90 to 180 degrees and the cartridge is all brass with O-rings and about 3 inches long, you likely need the Moen 1225 or its predecessor 1200. The Moen 1225 replaced the 1200 and is backward compatible. If you have a two-handle or three-handle Moen shower from the 70s or 80s, you may need the Moen 1224 cartridges. Take the old cartridge with you to the hardware store for exact matching, or take clear photos of the cartridge, handle, and valve body. You can also check Moen's website with your faucet model number found on the trim. Always buy genuine Moen cartridges for best fit and warranty, and consider buying the Moen cartridge puller tool for the 1222 as it often gets stuck.