Step-by-step fix
Step 1: Make sure the supply line is really the problem
- Look under the sink or behind the fixture and find the flexible line running from the shutoff valve to the faucet, toilet fill valve, or other fixture connection.
- Dry the area completely with a towel, then place a dry paper towel under both ends of the line.
- Turn the fixture on briefly if the shutoff valve is open, then watch for fresh water at the crimp, along the hose, or at either connection nut.
- Check that the leak is not coming from the shutoff valve body, the fixture itself, or water running down from above onto the line.
If it works: You have confirmed the fixture supply line or one of its end connections is the source of the leak.
If it doesn’t: If the line stays dry, keep tracing upward and around the shutoff valve. You may be dealing with a faucet leak, a loose valve packing nut, or condensation instead of a bad supply line.
Stop if:- The shutoff valve is leaking from the valve body or stem.
- The cabinet, wall, or floor shows swelling, rot, mold, or hidden water damage.
- The line serves hot water and the area is too hot to work around safely.
Step 2: Shut off the water and prep the area
- Turn the fixture shutoff valve clockwise until it stops. If there are separate hot and cold valves, close the one feeding the line you are replacing.
- Open the fixture to relieve pressure and let the remaining water drain out.
- Place a bucket or bowl under the supply line and lay down towels around the work area.
- Use the flashlight to confirm you can reach both ends of the line comfortably before loosening anything.
If it works: Water flow to that line is off, pressure is relieved, and the area is ready for a controlled disconnect.
If it doesn’t: If the shutoff valve does not fully stop the water, close the home's main water supply before continuing.
Stop if:- The shutoff valve will not turn, spins freely, or leaks heavily when you operate it.
- You cannot stop water flow enough to disconnect the line safely.
Step 3: Remove the old supply line and match the replacement
- Hold the shutoff valve steady if needed and loosen the supply line nut at the valve end with the adjustable wrench.
- Loosen the nut at the fixture end and remove the old line, letting any trapped water drain into the bucket.
- Take the old line with you as a reference and compare the new one for overall length, end sizes, and connection style.
- Choose a line that reaches naturally without being stretched tight, sharply bent, or forced into a twist.
If it works: The old line is out and you have a replacement that matches the original connections and fits the space properly.
If it doesn’t: If the new line does not match both ends exactly, stop and exchange it for the correct size and connection type.
Stop if:- The fixture threads or shutoff valve threads are cracked, badly corroded, or cross-threaded.
- Removing the old line exposes damage to the fixture shank or shutoff outlet.
Step 4: Install the new fixture supply line
- Start the new line by hand at the fixture end first, turning the nut carefully so it threads on smoothly.
- Start the valve-end nut by hand the same way. Make sure the line is routed naturally with no twist or kink.
- Tighten each connection snugly with the wrench after hand-threading is complete. Do not force the nuts or overtighten them.
- Wipe both connections and the line dry so any new leak will be easy to spot.
If it works: The new line is installed straight, untwisted, and snug at both ends.
If it doesn’t: If a nut will not thread on easily by hand, back it off and realign it before tightening again.
Stop if:- The nut binds immediately and appears to be cross-threading.
- The new line has to bend sharply or rub against a sharp edge to reach.
Step 5: Turn the water back on slowly and check for leaks
- Close the fixture if you opened it to relieve pressure.
- Turn the shutoff valve on slowly while watching the valve-end connection first, then the fixture-end connection.
- Let the line sit under pressure for a minute, then wipe both ends with a dry finger or paper towel to check for moisture.
- If you see a slight drip at a connection, tighten that nut a little more and check again.
If it works: Both ends stay dry with the line pressurized and the fixture shutoff valve open.
If it doesn’t: If a connection still drips after a small retightening, shut the water back off, disconnect that end, and inspect the fit and threads before trying again.
Stop if:- Water is spraying, not dripping, from a connection.
- The shutoff valve starts leaking from its own body or stem when reopened.
Step 6: Test the fixture in normal use
- Run the fixture for a minute or two the way you normally use it, then shut it off and inspect the line again.
- Check the cabinet floor or area below for any delayed drips.
- Look at the line routing one last time to make sure it is not rubbing, twisting, or touching something hot or sharp.
- Leave a dry paper towel under the connections for the next few hours and check it later.
If it works: The fixture works normally and the new supply line stays dry during use and after the water is shut off.
If it doesn’t: If the paper towel shows new moisture later, recheck both ends and the shutoff valve to confirm exactly where the water is coming from.
Stop if:- A slow leak keeps returning after careful retightening.
- The leak source turns out to be the shutoff valve, fixture body, or another hidden plumbing connection.
Replacement Parts
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FAQ
Do I need thread tape on a fixture supply line?
Usually no. Most fixture supply lines seal at a washer or built-in gasket, not on the threads themselves. Match the connection type and tighten it properly instead of adding tape unless the fitting specifically calls for it.
How tight should the supply line nuts be?
Hand-tight first, then snug with a wrench. They should be tight enough to seal but not forced. Overtightening can damage the washer, fitting, or threads and cause a leak.
Can I reuse the old supply line if it only leaked at the nut?
It is better to replace it if you already have it disconnected, especially if it is old, kinked, corroded, or has been leaking. Supply lines are inexpensive compared with the damage from a future failure.
What if the shutoff valve will not close all the way?
Shut off the home's main water before replacing the line. If the valve still leaks from the stem or body, the valve itself likely needs service or replacement before you can trust the repair.
How do I know what replacement length to buy?
Measure the old line or the distance between the shutoff valve and fixture connection, then choose a line that reaches comfortably without tension. Too short puts stress on the fittings, and too long can create awkward loops or kinks.