How to Fix a Leaking Under Sink Water Filter?

Author: Jason Hollow - Published: 2021/09/24 - Updated: 2022/11/22

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A pool of water near your under sink water filter indicates your system has developed a leak. If you’ve just spotted leakage, chances are, you are scratching your head standing next to it.

Now here is the tricky part! There could be multiple reasons for a leaking under sink filter. But, more often than not, a faulty O-ring is the culprit or, perhaps, a loose adapter. Not to worry, though; we have all the answers.

So, turn off the water supply to your filtration unit and grab a few towels to wipe down the water. Then take a deep breath and relax. By the end of this article, you’ll probably have figured out the issue and fixed it too. Ready to dive in?

So, here is our guide for fixing an under sink water filter that’s leaking!


Your under sink water filter might be leaking because:

  • A connection is loose – Tighten the connection with a wrench and ensure that any tubing is pushed all the way in. If it continues to leak, adapters or fittings might be broken and you need to replace them.
  • Filter housing is over-tightened – If the filter housing is damaged due to over-tightening, the damaged parts will have to replaced.
  • Filter cartridge is clogged – Replace.
  • Tank is damaged – You can just replace the whole tank or check and identify the broken components and replace them if possible.

How to Troubleshoot a Leaking Under Sink Water Filter

leaking under sink water filter thumbnail

Most people are baffled to see that their under sink water filter begin to leak right after a cartridge replacement.

This is because filter replacements require an understanding of the various components of these systems (see below for step by step guide). Once you educate yourself on how to replace filter cartridges, the task will become easy-breezy, and you won’t end up with a bigger mess to clean.

With that said, there can be multiple other reasons for a leaking filter. Therefore, before you attempt to fix it, you must be able to pinpoint the origin of the leak and diagnose the problem.

Of course, sometimes a leak is due to internal wear and tear, which is confusing to identify. In that case, you will need professional assistance. Contact a licensed plumber or carry the unit to a water filter supplier nearby.

Finding the Leak Source

To start, let’s address the most obvious question. Since under sink water filters are basically plumbed into your water supply, a leak could origin from anywhere in the unit or the connections. To begin, we shall troubleshoot the problem.

Here are the most common reasons that may be causing the leak.

  • Loose fittings, adapters or connectors
  • Incorrect installation using incompatible fittings
  • Fluctuating water pressure
  • Loose housings
  • Over-tightened housings
  • Broken or faulty O-rings
  • Damage from sediment or hard water
  • Clogged filters
  • Loose valves
  • Hot water damage

Origin of the Leak?

The thing about water leakages is that it’s challenging to figure out where they started from. Within minutes it’s all over the floor, and you are left staring at the unit. We got you covered, though.

Here are a few areas that are usually the trouble spots when dealing with under sink filter systems:

  • Top of the filter housings
  • Pressure release buttons
  • Connections
  • Tank
  • Faucet or dispenser

Jammed Filter Cartridge

Besides restricting the water flow rate, a jammed filter cartridge is one of the most common reasons for under sink filter leakage through the top of the housing. Since water enters the housing but is unable to move forward, it starts to make way from the top.

How to Fix

Under sink water filter cartridges should be replaced periodically depending on your water quality, daily usage and the type of system in use. Never skip or delay replacements, or you’ll end up with impure water and lengthy repair costs.

Loose Fittings

Quite simply, a loose nut, adapter or fitting allows water to leak through. It could either be due to fluctuating water pressure or just regular wear and tear. Carefully inspect all connections to figure out which one is leaking.

How to Fix

If a connector is loose, use plumber’s tape for added support and tighten it using a basic wrench. If it’s a quick-connect fitting, ensure the tubing slides all the way through inside.

However, if you spot a crack in any connectors or broken tubing, you must replace them with new ones.

Over-Tightened Filter Housing

Another common mistake most novice water filter owners make is over-tightening the filter housings.

They incorrectly assume that a tightly fitted cartridge will ensure a better seal. As a matter of fact, a watertight seal in plumbing fixtures is achieved by using gaskets and plumber’s tape, not by tightening the connections.

Not only does it make filter replacements a laborious job, but it also leads to cracks in the threads and the canister.

How to Fix

  • If the O-ring has been crushed, replacing it with a new one should suffice.
  • Cracked canisters must be replaced.

Damaged Tank

A water storage tank may leak due to multiple reasons. Generally, a hole in the tank, a ruptured air bladder or a faulty valve is to blame.

How to Fix

  • Reverse osmosis storage tanks should be re-pressurized at least once a year to prevent malfunctioning.
  • Water storage tanks can’t be easily fixed and probably will need to be replaced.

under sink reverse osmosis system with storage tank

Filter Cartridge and O-Ring Replacement

Most leakages in under sink water filters can easily be stopped by replacing the cartridges and O-rings properly. Here is a step-by-step guide to help you avoid common mistakes while replacing.

1. Close the Water Supply

Before you remove any filter housing, it is crucial to stop water inflow to your under sink filtration unit. Turn on the bypass valve (if applicable) or turn off the cold water supply.

2. Release Pressure

Next, remove leftover water from the unit by opening the faucet. Then, press and hold the pressure release buttons to remove excess air in the housings. Don’t skip this step, or you’ll end up with badly stuck sumps.

3. Remove Canisters

Simply twist the filter housings using your hands or a wrench to remove them from the cap.

4. Remove Filter Cartridges

Then, remove the filters using your fingers and inspect whether they’re usable or badly clogged. Make sure you remove old O-rings stuck to the bottom of cartridges or housings.

5. Replace O-Rings

An O-ring is a doughnut-shaped rubber used commonly in plumbing units to ensure a watertight seal. If it breaks or gets damaged, a leak is inevitable.

Remove the O-ring from its channel and inspect it. If it’s broken, inflexible, damaged or crushed, replace it with a new one. Remember to use food-grade silicone grease to lubricate.

At the same time, ensure that the channel is clean of debris or sediment.

6. Install New Filters

Slide in the new filter cartridges, ensuring they don’t tilt sideways.

7. Reassemble

Reassemble everything by twisting the housings back on. Make sure you don’t over-tighten.

8. Check for Leaks

Turn on the water supply to the under sink water filter and check if the leakage has stopped or not.

Tips to Prevent Leakages in Under Sink Water Filters

  • Always replace filters on time.
  • The O-ring and its channel should be inspected regularly at each filter change.
  • Only use silicone grease to lubricate the O-ring. Some people use vaseline, which does more harm than good.[1]
  • Don’t over-tighten or under-tighten the housings.
  • Only buy NSF certified high-quality filter replacements.


In conclusion, the first step to troubleshooting a leaking under sink water filter is finding the leak source.

Common culprits are loose fittings, clogged filter cartridges and worn-out O-rings among other things.

The good news is, most problems can be DIY-fixed even without much plumbing experience and time at hand.

In order to prevent issues with leaks in future, remember to follow our list of tips. Most importantly, always try to replace filter cartridges in due time.

Further Reading


Meet Jason Hollow

Jason Hollow Jason is the founder of Water Masterz and head of content creation. After six years in the industry, he has tremendous knowledge and first-hand experience on all things related to water treatment.

His credo: Not a single American should have to drink unhealthy water at home.

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