Help, My Under Sink Water Filter Is Leaking!

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Under sink water filters are advanced point-of-use home water filtration systems that allow you to have filtered water on demand. Depending on the quality of feed water at your place, you can install a single-stage, multi-stage, or reverse osmosis unit.

Most under-the-sink filtration systems will typically last for years without a glitch. However, some people come across a leaking under sink filter.

Unfortunately, a leak in the system may not only render it inoperable but could also lead to flooding and property damage. Don’t be intimated, though! We have all the solutions!

Keep reading to find out how to fix a leaking under sink water filter.

Key Takeaways

These are the most common causes why your under sink water filter is leaking and what you can do about it:

  • Loose connection – Use a wrench to tighten the connection and make sure to push any tubing all the way in. If water continues to leak, you may need to change broken adapters or fittings.
  • Over-tightened filter housing – Over-tightening a filter housing can cause damage beyond repair. You will need to replace the damaged parts.
  • Clogged filter cartridge – Water filters need to be replaced on time.  
  • Tank damage – Identify and replace broken components or buy a whole new tank.

How to Fix a Leaking Under Sink Water Filter

The first step towards fixing your under sink water filter is to identify the issue that has caused the leak.

Most people notice a leakage right after a filter replacement. It’s plausible that any of the fittings were not replaced correctly, or you accidentally nudged a connector.

If you cannot correctly diagnose the problem, you need to take your system to a water filter expert. You can also hire a licensed plumber to fix it for you.

Nevertheless, we believe most people can fix a leaking under sink filter by themselves. Here’s how:

Under Sink Reverse Osmosis System

Troubleshooting: Why Your Filter Is Leaking

As mentioned earlier, an under sink water filter could be leaking from practically anywhere on the unit. Therefore, identifying the problem is the most crucial step to fix it.

Under sink systems may leak due to:

  • Incorrect installation
  • Housing that is too loose or tight
  • Hot water damage
  • Sediment build-up on the O-ring channel
  • An incompatible adapter
  • A faulty O-ring
  • Loose valve
  • A sudden increase in water pressure

Where Is the Water Coming From?

You notice the cabinet under your kitchen sink is wet, and you begin to panic. You do a double-take and think about replacing the entire unit.

Relax, and try to pinpoint the origin of the leak. Use a flashlight or your mobile’s torch since under sink areas can get very dark, making it impossible to see properly. Common areas of trouble are:

  • Filter Housings
  • Pressure release buttons
  • Connections
  • Storage tanks
  • Clean water dispensers

Loose Connection

Over time, due to standard wear and tear, connections can get loose. At times, fluctuation in water pressure also results in loose or faulty connections. Identify the connection: the adapter or nut that is leaking.


Use a basic wrench and tighten the connection. If it’s a quick-connect fitting, make sure to push the tubing all the way in. If the water continues to leak, you will need to change the adapters or fittings.

Over-Tightened Filter Housing

An over-tightened filter housing is most often the culprit that leads to under sink water filter leakage. When the canister is tightened more than needed, it can get hairline cracks in the sump or on the threads.

Besides making it difficult to remove when replacing the filter, over-tightening a filter housing can crush the O-ring, causing it to leak.


  • If the O-ring has been damaged, replacing it will do the job.
  • If the housing is cracked or damaged, you need to buy a new one.

Clogged Filter Cartridge

A clogged filter will slow down the filtration process. A worn-out filter can also cause leaks due to damage caused by mineral deposits on the cartridge. It limits the amount of water flowing through, and eventually, the housing begins to leak.


Always replace under sink water filters timely, depending on your water quality and usage. If you have a hard water supply, consider installing a water softener to prevent further damage.

Tank Damage

Your water storage tank might be damaged which is causing the leakage. There could be a hole in it, one of the valves is malfunctioning, or the air bladder inside could be ruptured.


The only thing you can do is identifying and replacing the broken component or buying a whole new tank.

Changing the Filter Cartridge and/or the O-Ring

If your under sink water filter is leaking due to a clogged filter element or faulty O-ring, you must replace it immediately. Here is how to do it.

1. Turning Off the Water Supply

Turn off the cold water valve before attempting to disassemble the canisters. If you have a bypass valve on your filter, turn it on.

2. Releasing Pressure From Inside the Housing

Turn on the faucet and let the system drain out all water and air. Press the pressure release button on the filter head to depressurize the housing. If you don’t release the pressure from the housing, you will have a hard time opening the canister since it will be stuck.

3. Removing the Filter Sump

Use a plastic or metal wrench to remove the filter sump. Keep a small towel or a tub handy to catch any drips of water.

plumber replacing under sink water filter cartridge

4. Removing the Filter Cartridge

Slide-out the cartridge from the under sink water filter housing. Use a pair of nose-pliers to dislodge it if it’s stuck to the bottom. Make sure you remove the O-ring that is attached to the cartridge. At this point, you may give your housing a quick wash with soapy water to remove any dirt or debris.

5. Cleaning/Replacing the O-Ring

Run your finger along the O-ring channel to clean off any dirt. If the gasket is broken or inflexible, is full, or has nicks, replace it with a new one.

6. Installing a New Filter

Next, install the new cartridge making sure it fits the groove on the bottom of the sump.

7. Putting It Back Together

Twist the filter housing back to the system using your hands. If it doesn’t fit, turn the cartridge upside down. Finally, use a wrench to tighten it.

8. Checking for Leaks

Turn on the water supply and observe your under sink water filtration unit a couple of times within the next few hours for leaks.

What to Do After the Leak Is Fixed

  • Never use petroleum jelly on O-rings since it will break down the rubber, causing it to expand. Eventually, the seal will be compromised. Always use silicone-based lubricant for every cartridge replacement.
  • If you are using water from a well, you are probably dealing with a high level of sediment. You must clean the O-ring channel every time to ensure a water-tight seal.
  • Never over-tighten the filter housings.
  • Always use high-quality replacement cartridges.

If you have any questions about leaking under sink water filters please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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Leave a Comment:

ivanrolnick says May 28, 2022

My unit came with three extra o-rings rubber o-rings for the cartridges what do I do with them

    Gene says June 15, 2022

    Keep them as spare.

Upset says August 12, 2022

I had a Culligan EZ-1 filter installed for my kitchen sink. It worked great for years (changing filters regularly) but recently the filter housing split at the bottom, leaking and causing quite a bit of water damage. Has anyone else had this problem, and did Culligan do anything about it?

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