Fix a Leaky Faucet Cartridge

Cartridge faucets drip from a single point of failure: the cartridge itself, a sealed plastic-and-rubber valve assembly that controls water flow and temperature. Unlike compression faucets with their washers and seats, cartridge faucets have no user-serviceable parts—when they leak, you replace the entire cartridge. This is good news. The cartridge slides in and out of the faucet body like a magazine into a rifle, held by a single retaining clip or nut. No grinding, no valve seat dressing, no guessing about washer sizes. The challenge is identification. Faucet manufacturers each use proprietary cartridge designs, and a Moen cartridge will not fit a Delta body. You need to pull the old cartridge first, then match it at the hardware store or order by faucet model number. Expect to spend fifteen minutes disassembling, ten minutes at the store, and twenty minutes reassembling. Do this on a weekday evening and you will have a silent faucet by bedtime.

1. Kill the Water First. Turn off both hot and cold supply valves under the sink by rotating them fully clockwise. Open the faucet handle to release pressure and drain residual water from the lines. Leave the faucet in the open position throughout the repair.
2. Expose the Cartridge. Pry off the decorative cap on top of the handle using a flathead screwdriver or utility knife. Underneath you will find a Phillips or Allen screw—remove it and pull the handle straight up and off the cartridge stem. Set the screw in a cup so you do not lose it.
3. Free the Cartridge. Unscrew the dome-shaped trim sleeve or bonnet nut by hand or with adjustable pliers. Beneath it you will see the cartridge body and a small U-shaped or circular retaining clip holding the cartridge in place. Use needle-nose pliers to pull the clip straight out—it will resist slightly, then release.
4. Pull It Out. Grip the cartridge stem with pliers and pull straight up with steady, firm pressure. The cartridge should slide out of the faucet body. If it resists, rock it gently front to back while pulling—do not twist. Once removed, note the orientation of any flat edges or alignment tabs.
5. Get the Right Part. Take the old cartridge to a plumbing supply or hardware store and match it to a replacement. If you know your faucet brand and model, you can order the cartridge by number instead. Verify that the new cartridge has the same length, stem design, and port configuration as the old unit.
6. Seat the New Cartridge. Coat the rubber O-rings on the new cartridge with included silicone grease or plumber's grease. Align any flat edges or tabs on the cartridge with the corresponding slots in the faucet body, then push the cartridge straight down until it seats fully. The top of the cartridge should sit just below the faucet deck.
7. Put It Back Together. Push the retaining clip back into its slot until it snaps into place. Thread the trim sleeve or bonnet nut back on by hand, then snug it with pliers—do not overtighten. Slide the handle onto the cartridge stem, insert and tighten the set screw, and snap the decorative cap back on.
8. Turn Water Back On. Turn on the supply valves slowly and watch for leaks around the faucet base and cartridge. Open and close the faucet handle several times, checking both hot and cold operation. Let water run for thirty seconds to flush debris from the new cartridge.