Plumbing Under Bathroom Sink Leaking (Troubleshooting Guide)

On this page

Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

Turn off the water supply under the sink. Check for loose or corroded connections, worn-out washers, or damaged pipes. Tighten or replace faulty parts to stop the leak.

Confirm the symptom

Water pooling under the sink or dripping from pipes
Visible water stains or mold near the sink base
Dripping sound when the faucet is off
Low water pressure in the sink or nearby fixtures

Tools and prep

Adjustable wrench
Plumber's putty or Teflon tape
Replacement washers or O-rings
Bucket or towel to catch water
Pipe cutter (if replacing sections)
Turn off water supply before starting

Step-by-step troubleshooting

Shut Off Water Supply
Do: Locate and turn the shut-off valves under the sink clockwise to stop water flow.
Observe: Water stops flowing from the faucet.
Means: Water is safely off for repairs.
Next: Inspect connections
Inspect Connections
Do: Use a wrench to check for loose nuts or fittings on the supply lines and faucet base.
Observe: Leak stops when a connection is tightened.
Means: Loose fitting was the cause.
Next: Replace washers or O-rings
Replace Washers or O-rings
Do: Remove the faucet handle and inspect the washer or O-ring for wear. Replace if damaged.
Observe: Leak persists after replacement.
Means: Another issue may be present.
Next: Check for corroded or damaged pipes
Check for Corroded or Damaged Pipes
Do: Inspect copper or PEX pipes for cracks, corrosion, or pinhole leaks.
Observe: Visible damage or ongoing leak.
Means: Pipe section needs replacement.
Next: Replace damaged pipe section
Replace Damaged Pipe Section
Do: Cut out the damaged section with a pipe cutter. Install a new section using appropriate fittings and Teflon tape.
Observe: No leaks after reassembly.
Means: Repair successful.
Next: Test water supply
Test Water Supply
Do: Slowly turn the shut-off valves counterclockwise to restore water flow.
Observe: No new leaks and water pressure is normal.
Means: Issue resolved.
Next: Monitor for recurring leaks

Common causes

Cause	Likelihood	Difficulty	Part often needed
Loose or corroded connections Vibrations or mineral buildup can loosen fittings over time.	Common	Easy	Wrench, Teflon tape
Worn-out washers or O-rings Constant use causes rubber seals to degrade.	Common	Easy–Medium	Replacement washers/O-rings
Damaged or corroded pipes Copper pipes can corrode, and PEX can crack from freezing or UV exposure.	Sometimes	Medium	Pipe section, fittings
Faulty faucet cartridge Internal components can wear out, causing leaks even when the faucet is off.	Sometimes	Medium	Faucet cartridge
Improperly installed supply lines Incorrect assembly can lead to stress on joints and leaks.	Rare	Easy–Medium	Supply line kit
High water pressure Excessive pressure can strain connections and seals.	Rare	Hard	Pressure regulator

Parts matching

Common parts needed for repairs include:

Adjustable wrench for tightening connections
Plumber's putty or Teflon tape for sealing threads
Replacement washers or O-rings for faucet seals
Pipe cutter for removing damaged sections
New pipe sections or supply lines for replacements
Faucet cartridge if internal components are faulty

Purchase a basic plumbing repair kit for common tools and parts.

When to call a professional

Leak continues after replacing washers and O-rings
Significant pipe damage requiring soldering or complex repairs
Water pressure issues persist after fixing the leak
Mold or water damage is extensive
You're unsure about handling water supply lines or soldering

FAQ

Can I fix a sink leak without turning off the water?

No, always turn off the water supply to prevent flooding and ensure safety during repairs.

How often should I replace washers and O-rings?

Inspect annually or when leaks occur. Replace if worn or damaged.

What if tightening connections doesn't stop the leak?

Check for internal faucet issues or damaged pipes that may require part replacement.

Can high water pressure cause leaks under the sink?

Yes, excessive pressure can stress connections. Install a pressure regulator if needed.

Is it normal to see small drips after fixing the leak?

No, ensure all connections are tight and seals are intact for a complete fix.