The valve cartridge in your Moen faucet regulates the flow of water and controls the blend of hot and cold water. If the Moen faucet in your shower is leaking, the handle is hard to turn, or you can't turn the water all the way off the valve cartridge needs attention. There are many styles of Moen kitchen and bathroom faucets but the components of the faucets are standard with a few minor differences.
Shut off the water supply to the shower. Turn on the tap to release pressure in the water lines and to be certain the water is off. Older models of Moen faucets have plastic caps on the top of the faucet handle. New models have no plastic cap but have a hex nut on the underside of the handle.
Use a screwdriver to gently lift off the plastic cap and to remove the screw on the handle. Lift the handle off the faucet. Use an Allen wrench to unscrew the hex nut and remove the handle.
Use a screwdriver to remove the escutcheon (decorative trim) screws and lift it off the wall. Some Moen faucets have a handle adaptor and washers behind the handle. Use a screwdriver to remove adaptor screws and slide the adaptor and washers off the faucet.
Remove the retainer clip with needle nose pliers by pulling it straight up. The retainer clip is on the top front end of the valve cartridge. Until the retaining clip is removed the valve cartridge can't be removed.
Place a plastic removal tool on the valve cartridge. Use pliers to grab the removal tool and turn it from side to side to loosen the old valve cartridge. Remove the plastic removal tool and pull the valve cartridge straight out to remove it.
Wipe off the valve cartridge and use a brush to clean off corrosion. Examine the valve cartridge for corrosion and broken "O" rings. If no damage is present, apply a coat of silicone lubricant or petroleum jelly to the "O" rings.
Clean the cartridge housing on the faucet with a wire brush to remove corrosion or debris. Slide the valve cartridge in place, replace the retaining clip, and attach the faucet handle.
Turn on the water and test the faucet. If the problem is solved, remove the handle, install the escutcheon, handle adaptor, washers and the handle. If the problem is not gone, replace the worn valve cartridge with a new one.