A leaky shutoff valve under a bathroom sink is one of those small problems that can turn into a big mess if you ignore it. That little valve is what lets you turn off water to the faucet without shutting down the whole house. When it drips, sticks, or won’t close all the way, it can waste water, damage the cabinet, and make a simple faucet repair much harder. The good news is that replacing a shutoff valve is a realistic DIY job for many homeowners, especially if the pipe coming out of the wall is in good shape and you choose the right replacement valve.

Before you buy anything, turn off the main water supply to the house. Open the bathroom faucet to drain the lines and relieve pressure. Put a small bucket and an old towel under the valve, because some water will come out when you loosen the old connection. Take a close look at the pipe and the old valve. Most bathroom sink shutoff valves are angle stops, meaning they turn the water at a right angle from the wall to the supply line. You also need to know whether the pipe is copper, CPVC, or PEX, and whether the valve is attached with a compression fitting, a push-fit connection, or solder. That will tell you what kind of replacement will work.

If the old valve is a compression style, you will usually see a nut tightened onto the pipe. Hold the valve body with one wrench and turn the nut with another. Once the nut is loose, pull the old valve off the pipe. The compression ring, also called a ferrule, may stay stuck on the pipe. You can cut it off carefully with a small tubing cutter or a hacksaw, but protect the pipe so you do not gouge it. Wipe the pipe clean and smooth the end with fine sandpaper or emery cloth. A clean, square pipe end is important because the new valve needs a smooth surface to seal against.

For a push-fit valve, installation is even simpler. Cut the pipe square with a tubing cutter, then deburr the inside and outside so there are no sharp edges. Mark how deep the pipe should go into the valve, push the valve on firmly until it clicks or reaches the mark, and give it a gentle tug to confirm it is seated. Push-fit valves can work on copper, CPVC, and PEX, which makes them handy for homeowners who want to avoid soldering. Just make sure the pipe is fully inserted and cut cleanly, because a crooked cut is a common cause of leaks.

When you choose the new valve, match the pipe size and the outlet size. Many bathroom sink supply lines use a three-eighths-inch compression connection at the valve, while the pipe from the wall is often one-half inch. Bring the old valve or a clear photo to the hardware store if you are unsure. For compression fittings, slide the new nut and ferrule onto the pipe, insert the valve, and tighten the nut by hand. Then snug it with a wrench, usually another quarter to half turn. Do not crank down on it. Over-tightening can crush the ferrule or distort the valve and cause a leak. Teflon tape is for threaded connections, not for the compression ferrule itself.

Once the new valve is on, reconnect the supply line and turn the main water back on slowly. Open the faucet and let water run while you watch the valve and supply connection. A tiny seep at a compression nut may stop after a small additional turn, but do not force it. If a push-fit valve leaks, shut off the water and check that the pipe is cut square and inserted all the way. Let everything dry, then check again a day later. If the pipe is damaged, the joint is soldered inside the wall, or the old valve will not budge without twisting the pipe, it is wise to call a plumber. Knowing when to stop is part of a successful DIY repair.