Why does my dishwasher smell?

A drain smell and a dirty-machine smell are different problems — the drain side is usually the waste connection, the loop or the sink trap, and it is the one people never check.

Difficulty: beginner
Time: 40 min
Read: 4 min
Safety: caution

Short answer

Smell the empty machine cold, then smell the sink plughole. If both smell the same, the problem is in the shared waste, not in the dishwasher — check the high loop, the waste spigot and the sink trap. If only the machine smells, it is the filter, the sump, the door seal and the rubber gasket around the door edge.

Two quite different smells get described the same way. A sour, food-waste smell that gets stronger when you open the door is usually organic residue inside the machine. A drain smell — sewage, or a stale drain note — is waste gas coming back up the drain hose into the tub, and no amount of cleaning the machine will fix it. Establishing which one you have first saves you cleaning a machine that is already clean.

Safety

Never put bleach in a dishwasher with a stainless steel interior — it pits the steel and attacks the seals. Never combine bleach with any acidic descaler or vinegar; the mixture releases chlorine gas within seconds in an enclosed kitchen. Switch the machine off at the socket before reaching into the sump, where broken glass is common.

What you'll need

Dishwasher cleaner or citric acid
An old toothbrush and cloths
Rubber gloves
A screwdriver for the hose clip (optional)

Step by step

Work out which smell you have.Open the cold, empty machine and smell it. Then run the sink tap for ten seconds and smell the plughole. A matching drain note in both means the waste is the source. A sour food smell only inside the machine means the machine is the source.
For a drain smell, check the high loop first.The drain hose must rise to at least 600mm above floor level before it drops to the waste connection. Without that loop, waste water sits in the hose against the machine and drain gas travels straight up it. Clip the hose high inside the cabinet.
Check the waste connection and the sink trap.A hose pushed onto a spigot that was never properly sealed, or a trap that has been replaced with a shallow one, lets drain gas back into the room and the machine. A trap needs a working water seal; if the sink is rarely used, run it for thirty seconds a day.
Check whether the machine shares a waste with a washing machine.Two appliances into one waste is common and legitimate, but a poorly made junction backs greasy washing machine water into the dishwasher hose. If the smell is greasy and laundry-like, this is why.
For a machine smell, start with the filter and the sump.Twist the filter out, rinse and brush both parts under a hot tap, and look into the sump beneath with a torch. Grease and food sludge under the filter is the single largest source of smell inside the tub.
Clean the door seal and, more importantly, the lower door gasket.Fold the seal back and brush inside the channel. Then wipe the bottom edge of the door and the strip of tub below the seal — this narrow area is outside the wash zone, so nothing ever cleans it and it grows a black biofilm.
Clean the detergent dispenser and the rinse aid cap.Old detergent cakes in the corners of the dispenser and goes rancid. An old toothbrush and hot water.
Run an empty cycle at the highest temperature with a cleaner or citric acid.The hottest programme, with a proprietary dishwasher cleaner or 100-150g of citric acid in the base. This clears grease from the internal pipework and the spray arms, which is where the smell hides on a machine that only ever runs eco or quick cycles.
Change the habit that caused it.Machines that only run short or eco programmes never get hot enough to shift grease. Run one hot intensive cycle a month. Leave the door ajar between cycles so the tub dries. Do not leave a full machine sitting unrun for three days — use the cold pre-rinse programme instead.

Common mistakes

Pouring bleach into the machine — Bleach pits stainless steel tubs and degrades the door seal, and it does nothing about a drain-side smell. It is also the ingredient that makes a chlorine gas accident possible if a descaler is used afterwards.
Masking it with a scented freshener — The smell is organic matter or drain gas. A hanging deodoriser leaves both in place and the food washed in the machine still comes out smelling faintly of it.

If it doesn't work

Smell returns within a week of a thorough clean

Cause: It is a drain smell, not a machine smell — Fix: Go back to the high loop, the waste connection and the sink trap. Cleaning the tub cannot fix gas arriving from the waste.

Black slime around the bottom of the door

Cause: Biofilm in the strip below the door seal, which is outside the spray zone — Fix: Scrub it with a toothbrush and hot detergent solution. It needs doing by hand because no cycle reaches it.

Smell of burning or hot plastic

Cause: Not a hygiene problem — something plastic has fallen onto the heating element, or an electrical fault — Fix: Stop the cycle, switch off at the socket and look for melted plastic in the base. If nothing has fallen in, do not run it again — call an engineer.

Questions people ask

Should I leave the dishwasher door open between cycles?

Ajar, yes. A warm, sealed, damp tub is ideal for bacteria and mould. Most machines with auto-open doors do this automatically for exactly this reason. Leave it on the latch rather than swung fully out into the room.

Why does it smell worse in summer?

Warmth accelerates everything living in the residue, and drain traps evaporate faster in a warm house, so a rarely-used sink loses its water seal and lets gas through. Both effects are strongest in the same weeks.