Leaks only when the faucet is on
The cabinet stays dry until you run water, then a drip forms on the hose or under the faucet body.
Start here: Dry the hose and both ends, then run only hot and only cold to see which faucet supply hose wets first.
Faucet Leak Troubleshooting
Faucet Supply Line Leaking
Direct answer: A faucet supply line usually leaks from one of three spots: the top connection at the faucet, the bottom connection at the shutoff valve, or the faucet supply hose itself where the braid or tubing has failed. Start by drying everything completely and finding the first wet point, not the place where water finally drips.
Most likely: Most of the time this is a loose connection or a worn seal inside the faucet supply hose connection, especially if the leak shows up only when the faucet is turned on.
Get a towel and a flashlight under the sink, shut off the angle stops if water is actively dripping, and separate the leak pattern first. Reality check: water often runs down the hose and makes the lowest point look guilty. Common wrong move: wrapping supply line threads with tape when the connection actually seals with a washer or built-in gasket.
Don’t start with: Do not start by replacing the whole faucet or cranking down hard on the nuts. Overtightening can split a seal, twist the hose, or damage the shutoff valve.
Leak only when the faucet runs?Check the faucet supply hose and the top connection at the faucet first.
Leak even with the faucet off?Look hard at the shutoff valve connection and the valve body before blaming the faucet supply hose.
Last reviewed: 2026-04-05
What kind of faucet supply line leak do you have?
Leaks all the time, even with the faucet off
You see a steady drip or dampness under the sink without using the faucet.
Start here: Check the shutoff valve body and the bottom faucet supply hose connection before touching the faucet.
Water seems to come from the middle of the hose
The braided jacket or tubing looks damp along its length, not just at a fitting.
Start here: Wipe the hose dry and watch for a bead forming through the braid or at a kinked section.
Everything under the sink is wet and the source is unclear
The hose, valve, sink bottom, and cabinet floor are all damp.
Start here: Shut off both stops, dry every surface fully, then test one side at a time with a paper towel around each connection.
Most likely causes
1. Loose faucet supply hose connection
This is the most common cause when the leak starts after recent faucet work, cabinet cleaning, or bumping items under the sink.
Quick check: Use a dry paper towel at the top and bottom nuts. If only one fitting wets first, that connection is the problem area.
2. Failed seal inside the faucet supply hose end
Many faucet supply hoses seal with a rubber washer or built-in gasket, not thread tape. When that seal flattens or cracks, tightening only helps for a short time or not at all.
Quick check: If the nut is snug but water still forms right at the face of the connection, suspect the faucet supply hose end seal.
3. Damaged or kinked faucet supply hose
A hose that is twisted, sharply bent, rubbed through, or bulging can leak through the braid or split near a crimp.
Quick check: Look for a damp stripe, rust staining, frayed braid, or a bead of water forming on the hose itself.
4. Shutoff valve leak mistaken for a supply line leak
Water from the valve stem, packing nut, or valve body often runs onto the hose and cabinet floor, making the faucet supply hose look like the culprit.
Quick check: Dry the valve completely and watch the valve body and handle stem with the faucet off. If they wet up first, the leak is not the faucet supply hose.
Step-by-step fix
Step 1: Shut the water off if needed and find the first wet point
You need to know whether the leak is from the faucet supply hose, the faucet connection, or the shutoff valve before you touch anything.
- If water is actively dripping, close the hot and cold shutoff valves under the sink clockwise.
- Put a towel or shallow pan under the work area.
- Use a flashlight and dry the faucet supply hoses, shutoff valves, faucet shanks, sink bottom, and cabinet floor completely.
- Wrap a dry paper towel around the top faucet supply hose connection, then another around the bottom connection at the shutoff valve.
- Open the shutoff valves and watch for the first spot that turns the paper towel wet.
Next move: You now know which connection or component is actually leaking, and you can avoid replacing the wrong part. If everything gets wet too fast to tell, shut the valves again, dry it all, and test one side at a time by opening only the hot or only the cold shutoff valve.
What to conclude: A leak that starts at one fitting usually points to a loose connection or failed seal there. A leak that starts away from the fittings points to a damaged faucet supply hose. A leak that appears at the valve body points away from the faucet hose.
Stop if:
- The shutoff valve will not close fully.
- The valve body starts leaking more when you touch it.
- Water is entering the cabinet wall, floor, or a lower ceiling.
Step 2: Separate a faucet supply hose leak from a shutoff valve leak
A lot of homeowners replace the hose when the real leak is the valve stem or valve body right below it.
- With the faucet off, dry the shutoff valve body, handle stem, and the bottom faucet supply hose nut.
- Watch for 2 to 5 minutes without running the faucet.
- If the valve stays dry with the faucet off, run the faucet and watch again.
- If water appears only while the faucet runs, focus on the faucet supply hose or the top connection at the faucet.
- If water appears with the faucet off, look closely at the shutoff valve body and stem before blaming the hose.
Next move: You have separated a pressure-side valve leak from a use-only faucet supply hose leak. If the source is still unclear because water is tracking along the sink underside, dry the sink bottom and faucet body and check whether the leak is actually from the faucet base instead.
What to conclude: Use-only leaks usually point to the faucet supply hose or its upper connection. Constant leaks often point to the shutoff valve or the lower connection.
Step 3: Try a careful snug-up on the leaking faucet supply hose connection
A slightly loose connection is common and is the least invasive fix, but only if you tighten the correct nut without twisting the hose.
- Shut off the affected hot or cold shutoff valve.
- Hold the valve body or faucet shank steady with one wrench if needed.
- Use a second wrench to snug the leaking faucet supply hose nut just a small amount, usually about an eighth to a quarter turn.
- Do not twist the hose body and do not force the nut if it binds.
- Turn the water back on and watch the exact fitting again with a dry paper towel.
Next move: If the fitting stays dry through several on-off cycles, the leak was a loose connection and you are done. If the same fitting still leaks while the nut is already snug, the seal inside that faucet supply hose end is likely worn or the mating surface is damaged.
Step 4: Replace the faucet supply hose if the hose or end seal has failed
Once the hose body leaks or the end seal will not seal at a snug connection, replacement is the reliable repair.
- Shut off the affected shutoff valve and open the faucet to relieve pressure.
- Place a towel under the connection and disconnect the leaking faucet supply hose from the shutoff valve and faucet.
- Compare length, connector size, and end style before installing a new faucet supply hose.
- Route the new hose in a smooth curve with no sharp bend, twist, or rubbing point.
- Thread both ends by hand first, then snug the nuts without overtightening.
- Turn the water back on slowly and check both ends while the faucet is off and while it is running.
Next move: A dry hose and dry fittings during both idle and running tests confirm the old faucet supply hose or its end seal was the problem. If a new hose still leaks at the same spot, the mating surface on the faucet shank or shutoff valve may be damaged, or the leak source was misidentified.
Step 5: Finish with a pressure test and decide whether the next repair is the valve or faucet
You want to leave the cabinet dry and know whether the problem is solved or has moved to a different component.
- Dry the entire area one last time.
- Open both shutoff valves fully and leave the faucet off for several minutes.
- Run hot only, cold only, then both together while watching the repaired area.
- Check the cabinet floor again after 15 to 30 minutes for a fresh drip.
- If the faucet supply hose is dry but the shutoff valve or faucet base is wet, plan the next repair on that component instead of replacing more hoses.
A good result: If the hose, both connections, and the cabinet floor stay dry, the repair is complete.
If not: If the hose is dry but the valve leaks, the shutoff valve needs service or replacement. If the leak starts at the faucet body or base, move to a faucet leak diagnosis instead of chasing the supply line.
What to conclude: A clean pressure test confirms the fix. A dry hose with water elsewhere means the original symptom was masking a different leak source.
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FAQ
Can I use thread tape on a leaking faucet supply line?
Usually no. Most faucet supply hose connections seal with a rubber washer or built-in gasket, not the threads. If that seal is worn, tape will not fix it and can make the connection feel tight before it is actually sealed.
Why does the faucet supply line leak only when I turn the faucet on?
That usually points to the faucet supply hose itself or the upper connection at the faucet. Running water changes pressure and movement in the hose, so a weak seal or damaged hose often shows up only during use.
How tight should a faucet supply hose nut be?
Snug, not crushed. Start by hand, then tighten just enough to seal. If you have already snugged it and it still leaks, more force is usually the wrong fix.
Can a shutoff valve leak look like a faucet supply line leak?
Yes, all the time. Water from the valve stem or valve body can run onto the hose and drip from the lowest point, which makes the hose look bad when it is not.
Should I replace both faucet supply hoses if only one is leaking?
Not automatically. Replace the leaking faucet supply hose if diagnosis clearly points there. If the other hose is the same age and shows kinks, corrosion, or frayed braid, replacing both can make sense while you are already under the sink.
Is a small drip under the sink really urgent?
Yes. Small supply-side leaks can damage the cabinet, flooring, and ceiling below long before they look serious. A slow drip is still pressurized water, so it is worth fixing promptly.