Smelly Bathroom Sink: P-Trap Cleaning and Maintenance Guide
Top TLDR:
A smelly bathroom sink is most commonly caused by a dry or fouled P-trap — the curved pipe beneath the sink that seals out sewer gas by holding a small water reservoir. Lakeland and Polk County homeowners can fix a dry P-trap in 60 seconds by running the tap, and clean a dirty one in under 20 minutes with basic tools and no plumbing experience. If the smelly bathroom sink returns within a few days after P-trap cleaning, call S&S Waterworks — a deeper drain or venting issue is the likely cause.
The P-trap is one of the most important and least understood components in your bathroom. It sits beneath every sink, quietly doing one essential job: holding a small reservoir of water that physically blocks sewer gas from traveling up through the drain and into your home. When it fails at that job — whether because it ran dry, became coated in biofilm, cracked, or was improperly installed — the result is a smelly bathroom sink that no amount of surface cleaning will fix.
This guide covers exactly how the P-trap works, why it fails, how to clean it yourself, and when the problem goes beyond what a P-trap cleaning can address. If you want a broader overview of all the sources behind bathroom drain odors, start with our guide to eliminating drain odors and finding the source.
What the P-Trap Actually Does
Every drain fixture in your home — bathroom sinks, kitchen sinks, showers, tubs, and floor drains — has a P-trap or equivalent water seal. In bathroom sinks, the P-trap is the curved section of pipe below the drain opening, visible when you open the cabinet under the sink. The curve is deliberate: it retains a small pocket of water at all times, even when the sink isn't in use. That water pocket is a physical barrier. Sewer gas generated in the drain line below — hydrogen sulfide, methane, and other compounds — cannot pass through standing water, so it stays in the pipe where it belongs instead of entering your bathroom.
The P-trap also serves a secondary function: it catches small objects — rings, earrings, contact lenses — that fall down the drain, making them retrievable without having to open the wall. But the water seal is the primary engineering purpose, and it's why P-trap maintenance matters for both odor control and household safety.
Why P-Traps Fail and Cause Bathroom Sink Odors
P-traps fail to seal sewer gas for a limited number of reasons, each of which has a specific fix.
Evaporation from Infrequent Use
The most common cause of a smelly bathroom sink is the simplest: the trap water has evaporated. This happens faster than most homeowners expect, particularly in Polk County's warm climate where bathroom temperatures stay elevated year-round. In a guest bathroom, a vacation property, or any sink that isn't used regularly, the water in the P-trap can evaporate within a week to ten days during Florida's warmer months. Once it's gone, sewer gas rises freely through the drain opening into the room.
The smell from a dry P-trap is unmistakably sewage or sulfur — not musty or sour, but a sharp, raw odor that tends to appear suddenly rather than building gradually over time.
Fix: Run the cold water tap for 30 to 60 seconds. The trap refills and the odor stops within minutes. Add a tablespoon of mineral oil to the drain after running water in infrequently used sinks — the oil floats on the water surface and significantly slows evaporation. Make it a weekly habit to run all rarely used drains in your home for 30 seconds, including guest bathrooms, basement sinks, and utility room drains.
Biofilm and Organic Buildup Inside the Trap
In sinks used regularly, the P-trap doesn't dry out — but it does accumulate. Hair, soap scum, toothpaste, and skin cells pass through the drain and collect inside the curved section of the trap over time. That organic material feeds bacteria that produce hydrogen sulfide and other odor compounds. The smell from a biofilm-coated trap tends to be sour or musty — somewhere between mildew and sewage — and builds gradually rather than appearing overnight.
Biofilm buildup inside the trap also narrows the passage slightly, which can contribute to slower-than-normal drainage over time. If your bathroom sink both smells and drains more slowly than it used to, a fouled P-trap is the likely cause.
Fix: Clean the P-trap — full instructions in the step-by-step section below.
Trap Siphoning from Venting Problems
If the P-trap refills when you run water but empties again within a day or two, the trap is being siphoned dry by negative pressure in the drain line. This happens when the plumbing vent system — the pipes that run through the walls and up through the roof to equalize pressure and allow sewer gases to escape outside — is blocked or improperly configured. When vent pipes are blocked, running water in the drain line creates a suction effect that pulls the water out of the P-trap.
A siphoning trap produces the same sewage odor as a dry trap, but it comes back quickly after you refill it. You may also hear a gurgling sound in the drain as air is pulled through the water seal.
This is a venting problem, not a trap problem, and it requires professional diagnosis. A plumber can identify whether the issue is a blocked vent — common in Polk County's tree-heavy neighborhoods where debris accumulates on rooftop vent openings — or an improperly configured vent system.
A Cracked or Damaged Trap
PVC and ABS plastic traps can crack from impact, age-related brittleness, or the repeated stress of chemical drain cleaners. Metal traps — chrome-plated brass or galvanized steel common in older Lakeland homes — corrode and develop pinhole leaks over time. A cracked or leaking trap loses its water seal continuously, producing a persistent sewage odor that doesn't respond to cleaning or refilling.
Check the underside of the P-trap for moisture, mineral deposits, or visible cracks. Any sign of water around the trap means it needs to be replaced. P-trap replacement is a straightforward plumbing task that most homeowners can handle themselves, and it's also a quick, inexpensive professional repair if you'd rather have it done right the first time.
How to Clean a Bathroom Sink P-Trap: Step-by-Step
Cleaning the P-trap is a safe, effective DIY task that requires no special plumbing knowledge and only a few basic tools. As outlined in our DIY sewer maintenance guide, P-trap cleaning falls well within the range of tasks homeowners can safely handle themselves.
Tools needed:
A bucket (to catch the water that drains from the trap when you remove it)
Channel-lock pliers or slip-joint pliers
A bottle brush or flexible drain brush
Dish soap
An old towel or rag
Step 1: Clear the Cabinet and Position the Bucket
Clear everything out from under the sink. Place the bucket directly beneath the P-trap — the curved section of pipe below the drain. The trap holds water at all times, and it will drain into the bucket when you remove it.
Step 2: Loosen the Slip Joint Nuts
The P-trap is held in place by two slip joint nuts — one at the base of the drain tailpiece (the straight pipe coming down from the drain) and one where the trap connects to the drain stub-out in the wall. These are typically hand-tight plastic nuts on modern PVC traps, or metal nuts requiring pliers on older plumbing.
Turn both nuts counterclockwise to loosen. Start by hand; if they won't budge, use channel-lock pliers with gentle pressure. Don't overtighten when reinstalling — hand-tight plus a quarter turn with pliers is sufficient for PVC.
Step 3: Remove and Inspect the Trap
With both nuts loose, the P-trap section will drop free. Hold it over the bucket as you remove it — it holds a cup or two of water plus whatever has accumulated inside. Examine the inside of the trap: a moderately used trap will have a slimy dark coating; a heavily used or long-neglected trap may have a significant accumulation of hair and soap scum.
Also examine the interior of the drain tailpiece above and the stub-out in the wall. Any heavy buildup in these sections should be cleared as well while access is easy.
Step 4: Clean the Trap Thoroughly
Take the removed trap to another sink or outside. Use a bottle brush and dish soap to scrub the interior walls of the curved section, working the brush through from both ends. Rinse thoroughly with warm water. The goal is a clean interior surface with no slimy coating or hair remaining.
For particularly fouled traps, soaking the removed section in a mixture of hot water and a few tablespoons of baking soda for 15 minutes before scrubbing loosens stubborn buildup.
Step 5: Clean the Overflow Drain Opening
While the cabinet is open and you're already doing maintenance work, clean the overflow drain channel. The overflow is the small opening near the top rim of the bathroom sink basin — it prevents flooding if the drain is blocked and the tap is left running. The channel connecting that opening to the drain pipe accumulates biofilm just like the main drain, and it's almost never cleaned. Use a small flexible brush or pipe cleaner to scrub the overflow channel, then flush with a solution of baking soda dissolved in hot water.
Step 6: Reinstall the P-Trap
Slide the trap back onto the drain tailpiece and the wall stub-out. Hand-tighten both slip joint nuts, then snug them with pliers — a quarter turn past hand-tight for PVC, half a turn for metal. Don't overtighten, particularly on PVC, as the threads can strip.
Step 7: Test and Verify
Run the sink water for 30 seconds. Check under the sink immediately and again after a minute to confirm no leaks at either connection. If a slip joint is leaking slightly, tighten it gradually — usually a small additional turn solves it. Check once more an hour after installation.
Cleaning the P-Trap Without Removing It
If you prefer not to remove the trap, or if your under-sink space makes access difficult, a cleaning flush addresses moderate buildup without disassembly.
Pour half a cup of baking soda down the drain, followed by half a cup of white vinegar. Seal the drain opening immediately to trap the fizzing reaction inside the pipe. Wait 20 minutes. Follow with the hottest water your tap produces for three full minutes. Repeat monthly. This method is effective for light to moderate biofilm and is described in detail in the related guide on cleaning a sink drain naturally.
For established buildup, enzymatic drain cleaners applied before bed are more effective than the baking soda flush because they work biologically over several hours, digesting organic material inside the trap and the pipe rather than simply agitating the surface.
P-Trap Maintenance: Keeping the Smelly Bathroom Sink from Returning
A cleaned P-trap will stay clean longer with a few consistent habits.
Run all bathroom sinks for at least 30 seconds weekly, including any that aren't used daily. This prevents evaporation from breaking the water seal in infrequently used fixtures. For vacation homes and guest bathrooms in Lakeland and the wider Polk County area, this is the single most impactful maintenance step — P-trap evaporation is the number one cause of sudden drain odors in properties that sit unused between visits.
Clean the drain stopper monthly. The stopper collects hair and soap scum on its underside, and that material decomposes and contributes to both odors and the organic load entering the P-trap. Removing and scrubbing the stopper takes two minutes.
Apply enzymatic drain cleaner to bathroom sinks monthly. This keeps biofilm from establishing inside the trap and the drain pipe, so the P-trap cleaning intervals stay manageable. A monthly enzymatic treatment is one of the simplest and most cost-effective things a homeowner can do to prevent drain odors from developing.
Physically remove and clean the P-trap once a year, or whenever odors persist despite monthly flushing treatments. An annual hands-on clean keeps the trap in good condition and gives you the opportunity to inspect it for early signs of cracking or corrosion before they become leaks.
When P-Trap Cleaning Doesn't Fix the Smelly Bathroom Sink
If you've cleaned the P-trap and the smelly bathroom sink returns within a few days, or if the trap empties quickly after being refilled, the cause is beyond the trap itself.
A sewer vent blockage is the most common explanation for a trap that keeps going dry despite regular use. Vent pipes blocked by debris, bird nests, or leaf accumulation — all common in Polk County's tree-rich residential neighborhoods — create the negative pressure that siphons trap water. Clearing a blocked vent requires roof access and professional assessment.
If the odor is present at multiple bathroom fixtures simultaneously, or if other drains in the home are also affected, the source may be in the main sewer line. Tree root intrusion, common throughout Lakeland, Winter Haven, and Auburndale's established neighborhoods, allows sewer gas to migrate back through multiple fixtures in ways that no fixture-level cleaning will resolve. A video camera inspection of the sewer line identifies the exact cause quickly.
Specialized drain cleaning or hydro jetting addresses pipe-level buildup that's beyond what P-trap cleaning and home maintenance can reach. Professional hydro jetting scours pipe walls completely clean at pressures up to 4,000 PSI, removing years of accumulated material in a single service visit and producing results that hold far longer than any product-based treatment.
S&S Waterworks serves Lakeland, Winter Haven, Auburndale, Mulberry, and Bartow with professional drain cleaning, video inspection, hydro jetting, and full plumbing services. We offer upfront pricing, same-day availability, and a 100% satisfaction guarantee on all work. Book an appointment online or call (863) 362-1119.
Bottom TLDR:
A smelly bathroom sink is usually a P-trap problem — either dried out from infrequent use, coated in biofilm, siphoned by a vent issue, or cracked and leaking. Lakeland homeowners can clean a bathroom sink P-trap in under 20 minutes using basic tools, with monthly enzymatic treatment and weekly water runs keeping it odor-free long-term. If the smelly bathroom sink returns within days of cleaning the P-trap, call S&S Waterworks at (863) 362-1119 to diagnose the vent or main line issue behind it.