To repair a Moen bathroom faucet, first determine the problem. If the faucet is leaking, look closely at the fixture to determine where the leak is coming from. Many faucet leaks are caused by a scratched o-ring or grommet in the cartridge, according to the manufacturer. Shut off the water beneath the sink and turn on the taps to flush out any remaining water before you begin the repairs.
Assemble the tools necessary for faucet repair, including a screwdriver, Allen wrench, and slip joint pliers. You also need o-ring seals and a faucet repair kit. Determine the Moen model number and acquire the appropriate replacement parts before beginning the job. If the faucet leaks due to a damaged o-ring, replace the cartridge; replace both cartridges on a two-handle faucet.
Remove the faucet handle cap and use the Allen wrench to detach the faucet handle. Next, unscrew and remove the dome assembly, handle adapter and pivot stop. Use the slip joint pliers to remove the handle nut, and then pry the brass retainer clip loose with a screwdriver. Make special note of the cartridge's orientation; ensure that the replacement part is oriented the same way. Twist the spanner cap over the old cartridge to work it free, and put the new cartridge in its place.
Before reassembling the fixture, clear away any mineral deposits on the old parts using a solution of vinegar and water. Follow the above steps in reverse to put the faucet back together, and turn on the water supply. Test the assembly to make sure the leaks have stopped and the hot and cold taps produce the correct water temperature.