Low Water Pressure in One Faucet Only? Here Is Why
When one faucet runs weak and the rest of the house is fine, the restriction is at that faucet: the aerator, a hidden flow restrictor, a supply valve, or debris in the cartridge. How to find it in order, without buying parts you do not need.
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Short answer. If only one faucet is weak, the restriction is at that faucet. Check in this order: the aerator at the tip of the spout, the supply valves under the sink, the supply lines, then the cartridge. A clogged aerator is the most common cause and costs nothing to fix. A new cartridge is the answer only when water reaches the faucet at full strength and leaves it weak.
Check in this order
- The aerator. Unscrew it and run the faucet without it. Strong flow means the aerator was the problem: soak it in 50/50 vinegar and warm water, rinse, reinstall.
- The supply valves. Under the sink, make sure both are fully open. They often get left partly closed after a repair.
- The supply lines. Look for a kinked braided line. Disconnect it at the faucet into a bucket and crack the valve open briefly: strong flow means the line and valve are fine.
- The cartridge. If water reaches the faucet at full strength but comes out weak, the restriction is inside. Pull the cartridge and look for grit at the inlets on the bottom.
Read the flow
| What you see | What it usually means | What to do next |
|---|---|---|
| Strong flow with the aerator off | Clogged aerator or flow restrictor | Clean or replace the aerator |
| Weak on hot and cold, aerator clean | Supply valve partly closed, hidden restrictor, or a clogged cartridge | Open the valves, then check the cartridge |
| Weak on hot only, at this faucet only | Hot supply valve, or the hot side of the cartridge | Check that valve, then the cartridge |
| Weak on hot at every faucet | Upstream, often the water heater | Not a faucet part |
| Weak right after plumbing work | Loosened sediment caught downstream | Clean the aerator, then the cartridge inlets |
| Pull-down kitchen spray weak too | Clogged spray head, or a check valve in the hose | Clean the spray head screen |
Why the cartridge clogs
Sediment travels with the water and stops at the first narrow point. That is usually the aerator, then the small inlet ports on the bottom of the cartridge. Delta notes that on some models the flow restrictor sits under the cartridge or in the supply connection, so a clean aerator does not rule out a restriction at the faucet. Most cartridges cannot be taken apart, so one packed with grit is replaced.
When the cartridge is out, flush the valve body before the new one goes in. Cover the opening with a rag and a cup, and crack each supply valve open for a second. Otherwise the same grit lands in the new cartridge.
What to do next
- Shut off the supply and open the faucet.
- Pull the handle and cartridge, noting its orientation.
- Look at the inlets on the bottom for grit and scale.
- Flush the valve body, then fit the new cartridge, matched on splines, length, diameter and rotation.
Brand removal steps: Moen, Delta, Kohler, Pfister.
Cartridge out, but no idea what it is
Photograph it, including the bottom, and a specialist identifies the replacement. With a tape and the part in hand, you can also work it down yourself in the CID Finder.
When to call a plumber
If every faucet in the house is weak, the cause is in the main supply, a pressure reducing valve, or the water heater. Same if the supply valves under the sink are seized or leak when you turn them.
Common questions
Why does only one faucet have low water pressure?
Because the restriction is at that faucet. In order of likelihood: a clogged aerator or flow restrictor, a supply valve under the sink that is not fully open, a kinked or clogged supply line, and debris or scale inside the cartridge.
How do I clean a faucet aerator?
Unscrew it from the tip of the spout, take out the screen and insert, and soak them in a 50/50 mix of white vinegar and warm water to dissolve scale. Rinse and reinstall. Some faucets have a hidden aerator that needs a small key to remove.
The aerator is clean and it is still weak. What now?
On some faucets the flow restrictor is not at the spout. Delta notes it can sit in the hose connection under the sink or under the cartridge. Check the supply valves and lines next, then pull the cartridge and look for grit at its inlets.
Why did one faucet lose pressure right after plumbing work?
Work on the water lines shakes loose sediment and scale, which catches at the first narrow point downstream: the aerator, then the cartridge inlets. Clean the aerator first, then check the cartridge.
Can a bad cartridge cause low water pressure?
Yes. Grit can lodge in the cartridge inlets and scale narrows the passages. Many cartridges cannot be taken apart, so a clogged one is replaced rather than cleaned.