Bathroom Faucet Leaks: Cartridge, Washer, or Valve?

That drip-drip-drip from your bathroom faucet is annoying, sure. But it’s also telling you something specific about what’s wearing out inside.

A leaking bathroom faucet usually comes down to just three possible culprits: a worn cartridge, a deteriorated washer, or a bad valve seat. Reaching the right one depends on what kind of faucet you have and how it’s been treated over the years.

This guide walks you through each in plain language — what it sounds like, what causes it, and what a plumber would check first. You don’t need to take anything apart to follow along. And as always: If water is actively spraying, shut off the supply line first. That’s the one move you need to know before you do anything else.

The quick answer — what’s actually leaking?

A bathroom faucet leak almost always traces back to one of these three internal parts:

Cartridge leaks — most common in modern single-handle faucets. Water seeps out around the handle or drips from the spout even when the handle is off. The cartridge is a plastic or brass cylinder inside the faucet body that controls both hot and cold water. When the seals inside it wear out, water slips past.

Washer leaks — typical in older two-handle faucets with separate hot and cold knobs. The washer is a small rubber disc that presses against the valve seat to stop water flow. Over time it flattens, hardens, or splits. If your faucet drips from the spout and you have separate handles, a worn washer is the first thing to suspect.

Valve seat leaks — the valve seat is the metal surface the washer or cartridge presses against. If that surface gets rough or pitted, even a brand-new washer or cartridge won’t make a perfect seal. Water keeps sneaking past. This is often the hidden reason a repair doesn’t hold.

Where is the water coming from?

Before you dig into causes, look at where the leak shows up. That tells you a lot.

Dripping from the spout — this is the classic symptom. The seal isn’t holding. Single-handle faucet? Suspect the cartridge. Two-handle? Start with the washer on the side that’s dripping.

Leaking around the handle base — water pooling around the handle means internal O-rings or cartridge seals have failed. Common on single-handle faucets. The handle might also feel loose or wobbly.

Water under the sink — puddles in the cabinet usually mean loose supply line connections, a cracked supply line, or a failed faucet base gasket. That’s not a cartridge, washer, or valve seat issue — it’s a connection problem at the mounting point. If the faucet is not leaking but the fixture barely flows, use the low water pressure guide instead. If the leak seems to be traveling from above, compare it with the checks in what to do when water is leaking from the ceiling.

Dripping only with hot or only with cold — in a two-handle faucet, this narrows it down to the washer or valve seat on that specific side. If it drips regardless of which handle you use, the cartridge is the more likely culprit.

Cartridge leaks — the modern faucet problem

Most bathroom faucets made in the last 20 years use a cartridge — a self-contained unit that slides into the faucet body and controls water flow. There’s no washer to replace; you swap the whole cartridge.

What causes a cartridge to leak?

• Normal wear. The rubber seals inside dry out, crack, or lose their shape after years of use. Three to five years is a reasonable lifespan.
• Hard water sediment. Mineral deposits prevent the seals from sitting flush, especially in areas with hard tap water.
• Debris in the lines. Tiny particles of solder, Teflon tape, or pipe sediment can scratch internal sealing surfaces.
• Overtightening. Cranking the handle too hard can crack the cartridge housing or deform the seals.

How do you know it’s the cartridge? A single-handle faucet that drips from the spout when the handle is in the neutral (off) position. You might also feel resistance or grinding when you move the handle — that’s sediment or a broken internal piece.

The fix. A new cartridge costs $5–50 depending on brand and model. Moen, Delta, Kohler, and Pfister all use different cartridges, so you need the exact replacement. Most manufacturers have online lookup tools if you have the model number. Heads up: If the cartridge is seized inside the faucet body, a cartridge puller tool ($15 at a hardware store) helps. If it won’t budge and you’re using excessive force, you risk cracking the faucet body — turning a $20 repair into a $200+ replacement. That’s the moment to call a plumber and compare the repair line against how to read a plumbing estimate.

Washer leaks — the classic two-handle problem

Before cartridges became standard, most faucets used washers — simple rubber discs at the base of the valve stem. When you tighten the handle, the washer presses against the valve seat to stop water flow. When it wears out, water seeps past.

What causes a washer to leak?

• Aging and compression. The washer gets flattened from repeated tightening and can’t form a tight seal anymore.
• Hardening from heat. Hot water exposure makes rubber washers brittle. They lose flexibility and develop hairline cracks.
• Wrong size or type. If someone replaced the original with a slightly different washer, it won’t seal properly.

How do you know it’s the washer? If you have separate hot and cold handles and the faucet drips from the spout — especially with a steady, rhythmic drip… drip… drip — a worn washer is the most likely cause. If only the hot side drips, replace the washer on the hot side. Same for cold.

The fix. A pack of assorted washers costs about $3. But you need to match size and shape — take the old one with you. If the faucet is more than 15–20 years old, it might use a different washer type than modern ones.

Safety note: Older two-handle faucets (especially pre-1990s) may not have built-in shutoff valves under the sink. If yours doesn’t, or if the shutoffs are stuck, shut off water to the whole house at the main valve before disassembling anything. If you do not know where that is, start with how to find your main water shutoff valve. If water is actively spraying, shut off the supply line first.

Valve seat leaks — the hidden repeat offender

You replace the washer or cartridge, the leak stops for a week, then it comes right back. What happened?

The valve seat is the metal surface inside the faucet that the washer or cartridge presses against. Over time, that surface gets rough. Hard water erosion, corrosion, or abrasion from a worn washer can leave it pitted and grooved. No amount of new rubber will seal against a damaged metal surface.

How do you know it’s the valve seat? If you’ve replaced the washer or cartridge and the leak returns within days or weeks, suspect the valve seat. You can confirm by removing the faucet handle and stem and looking at the seat — if it feels rough or you see visible pitting, that’s your problem. Similar single-handle symptoms can show up at the shower, which is why the shower valve problems guide is worth reading if more than one fixture is acting up.

The fix. A valve seat grinding tool ($20–30) can smooth out minor pitting. For deeper damage, a replacement valve seat (if it’s a threaded insert) costs about $5–15. But here’s the honest truth: on many older faucets, the valve seat is part of the faucet body itself. You can’t replace it — you replace the entire faucet. If the seat is built-in and badly corroded, a new faucet is usually the more practical solution, especially if the faucet is 15+ years old. Ask whether the replacement faucet, supply lines, and labor are covered by the same workmanship terms described in what a good plumber warranty usually covers.

Faucet brands at a glance

The brand matters for replacement parts. Here’s the short version:

• Moen — Single-handle cartridge (Posi-Temp or 1225/1222). Cartridge seals and handle O-rings.
• Delta — Single-handle ball or cartridge. Ball assembly seats or cartridge O-rings.
• Kohler — Single-handle ceramic disc cartridge. Cartridge seals (especially with hard water).
• Pfister — Single-handle cartridge. Cartridge or supply line O-rings.
• American Standard — Two-handle washer or single-handle ceramic. Washer (two-handle) or ceramic disc seals (single).

If you don’t know the brand, look for a logo on the handle, base plate, or underside of the spout. No visible branding? Take photos from several angles — a plumbing supply house can usually identify it. If the old shutoffs or supply lines look corroded too, the broader material questions in PEX vs. copper pipes explained simply may help you understand what the plumber is describing.

How to check safely before you call

A plumber will want to know:

1. Faucet type — one handle or two? Brand if visible?
2. Leak location — spout, handle base, or under the sink?
3. Leak timing — constant? Only after use? Only with hot water?
4. Leak pattern — steady drip, trickle, or stream?
5. Faucet age — how old is it? Original to the house?
6. Recent work — any recent repairs or changes to water pressure?
7. Water type — hard water, well water, or city water?

Write these down and take a short video of the leak. A 15-second clip showing the drip pattern and the faucet type is more useful than a phone description. If the leak is part of a larger damage claim, compare your notes with when a plumbing leak is an insurance issue.

Questions to ask before any repair

If you are hiring the work out, pair these with the broader screening list in questions to ask before hiring a plumber.

• What exact replacement part does this faucet need — and how do I confirm compatibility?
• Is there a shutoff valve under the sink, and does it work?
• Is the valve seat damaged, or is it just the washer/cartridge?
• Is the faucet still under warranty? (Many brands offer limited lifetime warranties on cartridges.)
• If the cartridge is seized and breaks during removal, is the faucet still repairable?
• What’s the cost difference between repairing vs. replacing this faucet?

When to call a pro

Call a licensed plumber when:

• The shutoff valves under the sink don’t work. If you can’t isolate water to the faucet, any repair attempt risks a larger leak.
• The cartridge or stem is seized. Forcing it can crack the faucet body or damage the supply line connections.
• You’ve replaced the part and the leak came back. That usually means a valve seat issue or a problem deeper in the faucet body.
• The faucet is 20+ years old. Parts may be discontinued, and replacement is often the smarter investment.
• There’s water damage to the vanity or floor. You may need more than just a faucet fix.

You don’t need to diagnose everything yourself. A good plumber can identify the problem in about 60 seconds once the handle is off. Your job is to provide clear observations, a working shutoff (or the willingness to use the main valve), and accurate info about the faucet’s age and brand.

Frequently Asked Questions

How do I know if my faucet leak is a cartridge or a washer?

The faucet design tells you. Single-handle faucets almost always use a cartridge — no washers inside. Two-handle faucets (separate hot and cold) typically use washers. If you have a single-handle faucet dripping from the spout, it’s almost certainly the cartridge. If you have a two-handle faucet dripping from the spout, start with the washer on the side that’s leaking.

Can I replace a faucet cartridge myself?

If you can shut off the water, use a screwdriver, and follow a step-by-step guide, you can probably handle a basic cartridge swap. The tricky parts are: getting the exact replacement cartridge, removing a stuck cartridge without damaging the faucet body, and not losing small retaining clips or O-rings down the drain. If the cartridge is seized and won’t budge, stop and call a plumber.

Why did my faucet stop leaking after I jiggled the handle?

You temporarily shifted a worn seal back into place, but that’s not a fix. The leak will return — usually within a few hours or days. The material inside the cartridge or washer has already degraded. Plan on replacing the part soon.

Does hard water cause more faucet leaks?

Yes — absolutely. Hard water mineral deposits accelerate wear on cartridges, washers, and valve seats. Calcium and lime buildup prevents seals from sitting flush, scratches ceramic discs, and pits metal surfaces. If you live in a hard water area, expect to replace faucet cartridges more frequently — roughly every two to three years instead of every five to seven.

Should I replace the faucet or just fix the leak?

If the faucet is under 10 years old and parts are available, repair it. If it’s over 15 years old, has visible corrosion, or has gone through multiple repairs in the last few years, replacement is usually the better call. A new basic bathroom faucet costs $50–150 — not much more than a service call plus parts.

What’s the first thing I should do if water is spraying from under my sink?

If water is actively spraying, shut off the supply line first. Each supply line under the sink has its own small shutoff valve — turn it clockwise to close it. If those shutoffs don’t work or you can’t reach them, go to your main water shutoff valve (usually in the basement, crawlspace, or near the water meter) and shut off water to the whole house. Then call a plumber. Don’t try to tighten a spraying connection while water is under pressure — you can blow a fitting or flood the cabinet.

Bottom line

A bathroom faucet leak is rarely a mystery. Single-handle faucets leak from a bad cartridge. Two-handle faucets leak from a worn washer. And if you fix the part and the leak comes back, check the valve seat. That covers probably 95% of bathroom faucet leaks.

Start with safe observations. Know your faucet type. Shut off the water before you touch anything. And if a stuck part or a seized cartridge has you frustrated, that’s the right moment to call a pro — not the moment to grab a bigger wrench.

The goal isn’t to make you a plumber. It’s to give you enough understanding to describe the problem clearly and make an informed decision about what comes next. A little knowledge before you pick up the phone saves time, money, and a lot of unnecessary stress.