The first and most critical step is to shut off the water supply. Since we're assuming there is no dedicated shutoff for the shower, you'll need to locate the main water shutoff valve for your entire home. This is typically found in a basement, crawlspace, or utility closet where the water service line enters the house. Turn the valve handle clockwise until it is fully closed. To ensure the water is off and to relieve pressure, open the faucet in the shower and a faucet on a lower floor (like a basement sink) to drain the remaining water from the pipes. Keep them open until the water stops flowing.

With the water off, you can begin disassembling the faucet. First, locate the set screw that secures the handle to the valve stem. On many single-handle Moen faucets, this screw is an Allen screw located in a small hole on the underside of the handle. Use the appropriate size from an Allen wrench set to turn the screw counter-clockwise. You do not need to remove the screw completely, just loosen it enough so the handle can be pulled straight off the valve stem. If the handle is stuck due to corrosion, you may need to gently wiggle it back and forth while pulling. If there's a decorative cap on the front of the handle, use a small flathead screwdriver to gently pry it off to access a Phillips screw underneath.

After removing the handle, you'll see a sleeve (often black plastic) and a large decorative plate called an escutcheon. First, pull the sleeve straight off; it may have a temperature limit stop on it that also pulls off. The escutcheon plate is typically held to the wall with two long Phillips head screws. Use a Phillips screwdriver to remove these screws. If the edge of the escutcheon was sealed with caulk, you may need to carefully run a utility knife around the perimeter to break the seal before pulling it away from the wall. Behind the plate, you will see the main valve body embedded in the wall.

Now you can see the valve cartridge. The Moen cartridge is held in place by a small U-shaped retaining clip inserted into the top of the valve body. Use needle-nose pliers or a small screwdriver to grab the top of the clip and pull it straight up and out. Keep this clip in a safe place. With the clip removed, the cartridge can be extracted. Often, they are very stuck. Your replacement Moen cartridge (like the 1222 or 1225 model) should come with a small plastic tool. Insert this tool over the cartridge stem and rotate it back and forth to break the cartridge free. Then, use a dedicated Moen cartridge puller tool. This metal tool screws onto the cartridge stem and uses leverage to pull the cartridge straight out of the valve body without damaging the housing. Pull firmly and steadily until the cartridge comes free.

Once the old cartridge is out, inspect the inside of the brass valve body. Use a rag and a small brush (an old toothbrush works well) to clean out any mineral deposits, debris, or pieces of old rubber seals. The inside surface must be clean and smooth for the new cartridge to seal properly. Take your new cartridge and apply a thin layer of the included plumber's grease (silicone grease) to all the rubber O-rings. This lubricates them and helps ensure a watertight seal. Align the new cartridge with the 'H/C' marking facing up, just as the old one was oriented. Push the new cartridge firmly and evenly into the valve body until it is fully seated. It should slide in until its flange is flush with the front of the valve body.

Reassemble the faucet components in the reverse order of disassembly. Start by re-inserting the U-shaped retaining clip into the slot at the top of the valve body, ensuring it snaps securely in place over the new cartridge. Next, slide the faucet sleeve back over the valve body, followed by the escutcheon plate. Secure the escutcheon plate with its two screws, being careful not to overtighten and crack the plate. Finally, push the handle back onto the valve stem, aligning it properly, and tighten the set screw with your Allen wrench or screwdriver to secure it. Pop any decorative cap back on if your model has one.

With the faucet fully reassembled, you can now test your work. Ensure the shower faucet is in the 'off' position. Go back to your main water shutoff valve and turn it on slowly, counter-clockwise, until it is fully open. This prevents a sudden pressure surge (water hammer). Return to the bathroom and closely inspect the area around and behind the faucet handle. Use a dry paper towel to wipe the area and check for any moisture. If it remains dry, turn the faucet on and cycle it between hot and cold. Check for leaks again while the water is running, then turn it off and check a final time. If there are no leaks, the repair is complete.