Why Does My Washing Machine Smell Like Sewage?

Let's Narrow This Down

I know this smell is unpleasant -- it can make your whole laundry room (and sometimes your whole house) feel like something is seriously wrong. But the cause is usually something straightforward once you know where to look. Let's work through the possibilities together, starting with the most common culprits.

The first question to ask yourself is: when does the smell happen?

• If you smell it all the time, even when the machine is not running, the issue is almost certainly sewer gas coming back through the drain. That points to the P-trap or the drain hose connection.
• If you smell it mainly when you open the machine or during a cycle, the problem is more likely inside the machine -- bacteria, mold, or trapped debris.
• If it smells only when the machine is draining, the drain line itself may be partially clogged, and stagnant water in the pipe is the source.

The P-Trap Problem

Every drain in your house has a P-trap -- that U-shaped bend in the pipe that holds a small amount of water. That standing water acts as a seal, preventing sewer gas from flowing back up through the drain and into your home. It is a simple and elegant system, but it depends on that water staying in place.

If your washing machine's drain has a P-trap that has dried out (common if you have not used the machine for a while, or if it is in a vacation home), sewer gas has a clear path straight into your laundry room. This is the same mechanism that causes a rotten egg smell from sink and shower drains.

The fix: Run water into the standpipe or drain to refill the P-trap. Simply running a short wash cycle will usually do it. If the machine drains into a utility sink, run the sink faucet for 30 seconds.

If you use your machine regularly and still suspect the P-trap, there may not be one installed at all. Some older installations skip the P-trap, which is a code violation but not uncommon in DIY plumbing. Look at the drain pipe behind or beneath the machine. You should see a U-shaped curve. If the pipe goes straight into the wall or floor without that curve, you need a plumber to add one.

The Drain Hose Connection

This is the one that catches a lot of people by surprise. The way your washing machine's drain hose connects to the standpipe matters quite a bit, and there are two common mistakes.

Hose pushed too far into the standpipe. The drain hose should go into the standpipe only about 6 to 8 inches. If it is shoved all the way down, it can bypass the P-trap or create a siphon effect that pulls the water out of the trap. Pull the hose out until only 6 to 8 inches are inside the pipe.

No air gap. There should be a small gap between the drain hose and the standpipe -- the hose should not be sealed or taped in place. This air gap prevents siphoning. If someone sealed the connection with tape or a tight fitting, remove it and leave a gap.

No high loop. If the drain hose runs directly from the machine to the standpipe at a low point, dirty water can flow back into the machine. The hose should loop up to at least the height of the machine's top (or higher) before going down into the standpipe. Many machines have a hook or clip on the back to hold the hose up. If yours does not, secure the hose to the wall with a hose clip at the right height.

Mold and Bacteria Inside the Machine

If the sewer smell is coming from inside the drum rather than from the drain, you are dealing with bacteria or mold that have built up over time. This is especially common with front-load washing machines, but it can happen with top-loaders too.

The warm, damp environment inside a washing machine is a perfect breeding ground for bacteria and mold. They feed on soap residue, fabric softener, body oils, and lint that accumulate in areas you cannot easily see -- under the rubber door gasket, inside the pump filter, and in the detergent dispenser.

Cleaning Out the Machine

Preventing the Smell From Coming Back

Once you have eliminated the current smell, a few habits will keep it from returning:

• Leave the door open after every wash. If you have pets or small children and cannot leave it wide open, even cracking it an inch helps air circulate.
• Use the right amount of detergent. Excess detergent does not rinse out completely and becomes food for bacteria. If you have a high-efficiency (HE) machine, use HE detergent and follow the measuring guidelines. Most people use two to three times more detergent than necessary.
• Skip the liquid fabric softener. It leaves a waxy residue inside the machine that bacteria love. Dryer sheets or wool dryer balls are alternatives that do not affect the washer.
• Run a monthly cleaning cycle. Even if there is no smell, a monthly hot cycle with vinegar or a commercial washer cleaner (like Affresh) keeps things fresh.
• Use hot water occasionally. If you wash everything in cold water (which is fine for most laundry), bacteria can build up because they are never killed off. Running one hot cycle per week or per month helps.

When to Call a Plumber

If you have checked the P-trap, fixed the drain hose, and cleaned the machine thoroughly but the sewage smell persists, the issue may be deeper in your plumbing:

• A partially blocked sewer vent on your roof can cause slow draining and sewer gas backup throughout your home. If multiple drains in your house smell or gurgle, this is a likely cause.
• A cracked or damaged drain pipe inside the wall can leak sewer gas even if the P-trap is intact.
• A main sewer line issue can push gas back through the lowest or most poorly sealed drains in the house, and the washing machine standpipe is a common weak point.

These are situations where a plumber with a camera scope can diagnose the issue accurately. But try the simpler fixes first -- the overwhelming majority of washing machine sewage smells come from the three causes we covered above.

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