Why Is Water Leaking Under My Kitchen Sink Only When the Faucet Runs? A Complete Troubleshooting Guide
You turn on your kitchen faucet to fill a pot for pasta, and a few seconds later, you spot water pooling on the cabinet floor. But when you check again later with the faucet off, everything stays dry.
This intermittent leak is frustrating—and confusing. If water only appears when the faucet is running, the problem isn’t your drain pipes. It’s almost always a supply line issue, a faulty faucet component, or a loose connection that only reveals itself under water pressure. This guide walks you through the causes, shows you the best way to find the leak, and gives you step-by-step solutions to fix it yourself.
TL;DR
A leak that only happens when the faucet runs usually points to a problem with the water supply lines, not the drain. Common causes include loose compression fittings, cracked supply hoses, a worn faucet O-ring or cartridge, or a failing sprayer diverter. The fix starts with identifying exactly where the water comes from—dry everything, run the faucet, and follow the drip. Most repairs involve tightening connections, replacing supply hoses, or rebuilding the faucet with new washers and O-rings.
Key Takeaways
- Supply lines are the usual suspects: The flexible hoses that bring water to your faucet can develop pinhole leaks or loose connections that only drip under pressure.
- Check the faucet body itself: A worn O-ring or cartridge inside the faucet can leak internally, sending water down the outside of the supply lines.
- Don’t ignore the sprayer diverter: If you have a pull-out sprayer, a failing diverter can leak when water pressure shifts.
- The drip location tells the story: Water running down the outside of a supply hose means a leak above that point. Water pooling near the base of the faucet points to internal faucet wear.
- Always turn off the water supply before disassembling anything: Locate the shut-off valves under your sink and close them before you start any repair.
The Real Causes Behind a Leak That Only Happens When the Faucet Is On
When your kitchen sink only leaks during active use, the water pressure is the trigger. Here are the most common reasons why.
Why Water Appears Under Your Sink Only When the Faucet Runs
Loose or Damaged Supply Line Connections
Your faucet connects to your home’s water supply through flexible braided hoses or rigid pipes. These connections use compression fittings that rely on tight seals. Over time, the nuts can loosen from vibration or thermal expansion. When the water is off, there’s no pressure, so no leak. Turn the faucet on, and pressurized water seeps through the tiny gap. This is one of the easiest problems to fix—often just a quarter-turn with a wrench solves it.
Pinhole Leaks in Supply Hoses
Braided stainless steel supply hoses can develop tiny pinhole leaks from corrosion, abrasion, or manufacturing defects. These microscopic holes only leak when water flows through the hose. The leak may be so small that it sprays a fine mist rather than a steady drip, making it tricky to spot.
Worn O-Rings or Cartridge Inside the Faucet
Modern faucets use O-rings and cartridges to control water flow and prevent leaks. When these rubber components wear out, water can escape from inside the faucet body. This water then travels down the outside of the supply lines, pooling under your sink. Always unplug appliances before cleaning or repairing them, but for faucet repairs, your first step is always turning off the water supply valves.
Faulty Sprayer Diverter
If your kitchen sink has a pull-out or side sprayer, a diverter valve directs water between the main faucet spout and the sprayer. When this diverter fails, water can leak back through the sprayer hose, or it can leak from the diverter body itself, often only when the faucet runs.
Cracked Faucet Base or Mounting Nut
The faucet mounts through a hole in your sink or countertop. The mounting nut underneath can loosen, and the base may develop cracks from overtightening or age. When water runs, it can seep through these gaps and drip below.
How to Fix a Leak That Only Happens When the Faucet Runs: Step-by-Step
Before you begin, gather these supplies:
- Adjustable wrench or basin wrench
- Flashlight
- Dry towels or paper towels
- Replacement supply hoses (if needed)
- Replacement O-rings or cartridge (match your faucet brand)
- Plumber’s tape (PTFE tape)
- Small mirror (helps see behind pipes)
Step 1: Find the Exact Leak Source
This is the most critical step. Place a bucket under the pipes to catch drips and prevent water damage while you work.
- Dry everything under your sink—the supply lines, the shut-off valves, the faucet base, and all connections.
- Lay dry paper towels flat under each potential leak point: under the supply line connections, under the faucet base, and along the drain pipes.
- Turn on the faucet at full pressure and let it run for a minute.
- Immediately grab your flashlight and look closely. The wet spot on your paper towels tells you exactly where the leak originates.
Key observation: If water is running down the outside of a supply hose, the leak is above that point—likely at the faucet connection or inside the faucet itself. If water drips directly from the connection nut, that’s where the problem lies.
Step 2: Tighten Supply Line Connections
If you spot moisture at the connection points where supply lines meet the shut-off valves or the faucet shanks:
- Use your wrench to gently tighten the compression nuts. A snug quarter-turn often stops the leak.
- Be careful not to overtighten—you can strip threads or crack brass fittings. Hand-tight plus a gentle turn with a wrench is sufficient.
If tightening fails, move to the next step.
Step 3: Replace Supply Hoses
If you see water leaking from the middle of a hose or if tightening doesn’t stop the drip at connections:
- Turn off both hot and cold water shut-off valves under the sink. If you don’t have shut-off valves, you’ll need to turn off the main water supply to your home.
- Place a bucket under the connections.
- Use your wrench to disconnect the supply hoses from both the shut-off valves and the faucet shanks.
- Take the old hoses to a hardware store to buy exact replacements. They come in standard lengths like 12, 16, or 20 inches.
- Install new hoses, ensuring the rubber washers inside the ends are seated properly. Do not use plumber’s tape on these compression fittings—the rubber washer creates the seal, not the threads.
- Tighten securely but not excessively, turn the water back on, and check for leaks.
Step 4: Inspect the Faucet O-Rings and Cartridge
If your supply lines are dry and the leak seems to come from above them, the faucet itself is the problem.
- Turn off the water supply valves before proceeding.
- Remove the faucet handle. This usually involves prying off a decorative cap and unscrewing a small setscrew.
- Pull out the cartridge or ceramic disc assembly. These parts vary by brand—note the orientation and how everything fits.
- Inspect the O-rings (small rubber rings) for cracks, flat spots, or deformation. Replace any damaged O-rings with matching ones from a hardware store or a manufacturer-specific repair kit.
- If the cartridge itself is cracked or corroded, replace the entire cartridge.
- Reassemble in reverse order and test.
Step 5: Check the Sprayer Diverter
If you have a pull-out sprayer and the leak coincides with using it, the diverter may be failing.
- The diverter is usually located in the faucet body or at the base where the sprayer hose connects.
- Consult your faucet’s manual for diverter location and replacement instructions.
- In many cases, replacing the diverter is as simple as unscrewing it and installing a new one.
- Regular maintenance of your faucet components prevents unexpected leaks and extends the life of your fixtures.
Step 6: Tighten the Faucet Mounting Nut
If water seems to appear from under the entire faucet base:
- Look up under the sink at the large locknut or mounting bracket holding the faucet to the sink deck.
- Use a basin wrench to tighten it. The basin wrench is designed specifically for this task because of its awkward angle.
- If the base has a visible gap, tightening the nut often seals it.
Simple Tips to Prevent Future Leaks
- Inspect supply hoses every six months: Look for bulges, kinks, or rust spots. Replace any hose that shows signs of wear.
- Know where your shut-off valves are: In an emergency, stopping water flow quickly prevents major damage. Test your valves occasionally to ensure they still work.
- Don’t use the faucet handle as a lever: Putting heavy pressure on the handle can stress internal components and cause leaks.
- Replace washers and O-rings proactively: If your faucet is over five years old, consider rebuilding it with new seals before it fails.
When to Call a Professional Plumber
Some faucet repairs require specialized tools and expertise. Call a plumber if:
- The leak persists after you’ve tried all these steps. A deeper issue may exist inside the wall.
- You don’t have shut-off valves. You’ll need to shut off the main water supply, and not all homeowners feel comfortable doing that.
- The faucet is old and replacement parts are unavailable. A plumber can help you choose and install a new faucet.
- You see water damage inside the cabinet walls. This could indicate a leak that has been ongoing for a while and may require mold remediation.
“Most intermittent leaks under a kitchen sink are from easily replaceable parts—supply hoses, O-rings, or cartridges. The trick is finding the source, and drying everything first is the best way to do it.”
Frequently Asked Questions (FAQs)
1. Why does water leak under my sink only when the faucet is on?
The water pressure from running the faucet forces water through a tiny gap or pinhole that stays sealed when the water is off. The source is almost always a supply line, a faucet component, or the sprayer diverter.
2. Can a loose aerator cause a leak under the sink?
No. A loose aerator leaks at the faucet spout above the sink. Leaks under the sink come from below the counter level.
3. How do I tell if the leak is from a supply hose or the faucet itself?
Dry everything, run the water, and watch carefully. If water runs down the outside of a hose, the leak is above that point—likely the faucet. If water drips directly from a connection nut, that’s the leak source.
4. Should I use plumber’s tape on supply line connections?
No. Supply lines use rubber washers to create a watertight seal. Plumber’s tape on threads can actually prevent the washer from seating correctly and cause more leaks.
5. My faucet leaks only when I use the sprayer—why?
That’s a classic sign of a failing diverter valve. The diverter isn’t sealing properly, allowing water to escape where it shouldn’t.
6. How long do kitchen supply hoses last?
Most manufacturers recommend replacing braided supply hoses every 5 to 10 years. They’re inexpensive and replacing them proactively prevents sudden flooding.
7. Is it safe to tighten plumbing connections with pliers?
Use an adjustable wrench or a basin wrench. Pliers can slip and damage fittings. A basin wrench is designed specifically for tight spaces under sinks.
References
Trusted Sources for Kitchen Faucet and Supply Line Repairs:
- DIY Network – How to Repair a Leaky Kitchen Faucet
- Family Handyman – How to Fix a Leaking Faucet
- Bob Vila – Leaking Kitchen Faucet Repair Guide
- Home Depot – Kitchen Faucet Repair Tips
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