1. The carbon filter is saturated
Cause of most cases, and the cheapest to fix. The unit does not vent to the outside. It vents through a small activated-carbon filter that holds the odour back. Carbon saturates. Once it does, every flush pushes air straight out of the vent and into the room.
How to tell: the smell arrives during or just after a flush and fades between uses. It has got gradually worse over months rather than appearing overnight.
Fix: replace the filter. It clips into the vent on top of the housing and takes about two minutes with the power off. Ours cost a few euro.
Replace it once a year as routine, whether it smells or not. Almost nobody does, which is why this is the number one cause.
2. The vent is blocked or has been sealed over
The same vent has to breathe. Two things stop it. Someone tiles, boxes in or foams around the unit and covers the vent, usually during a bathroom refit. Or the filter has been soaked: if the discharge pipe ever backed up, water reaches the carbon, and wet carbon neither filters nor passes air.
How to tell: the pump sounds like it is straining, the tank empties slowly, or the smell started right after building work.
Fix: clear whatever covers the vent and give the unit air. If the carbon is damp, replace it; drying it out does not restore it. If the pipe backs up repeatedly, the real problem is item three or the discharge run itself.
3. The non-return valve is letting waste water back
The valve at the outlet stops what has been pumped up the pipe from running back down when the motor stops. When it fails, a little comes back every cycle and sits in the tank between uses, going stale.
How to tell: the smell is constant rather than tied to flushing, and worse in the morning after a night of no use. You may hear the pump start briefly on its own with nobody using the bathroom, which is the returning water tripping the float switch.
Fix: a new non-return valve. It is a wear part, it costs little, and it fits at the outlet elbow. If the pump also keeps starting by itself, replace the valve before you suspect anything electrical.
4. A trap has dried out or was never fitted
This one is not the macerator at all, which is why people replace a perfectly good pump over it. Every fixture draining into the unit needs its own water trap. A shower or basin used rarely, in a guest bathroom or a holiday home, loses its trap water to evaporation in a few weeks, and the tank then vents straight up through the empty trap into the room.
How to tell: the smell is strongest at one particular fixture rather than at the machine, and it is worse after the room has been unused.
Fix: run water at each fixture for thirty seconds. If the smell goes, that was it. In a rarely used room, run the taps monthly or fit a waterless trap valve.
5. Biofilm inside the tank
The slow one. Fats, soap and paper leave a film on the tank wall and under the lid that no amount of flushing removes. It is normal in an older unit and it does smell.
How to tell: everything above has been ruled out, the unit is several years old, and the smell is faint but permanent.
Fix: a descaling and cleaning cycle. Cut the power, run the descaler through per the maintenance page, leave it to stand, then flush through twice. Do not use bleach or drain cleaner: they attack the seals and the impeller and will cost you more than the smell.
When to stop and call someone
Stop and get help if the smell comes with waste water appearing anywhere it should not, if the unit trips the electrics, or if it hums without pumping. Those are not odour problems and the troubleshooting table covers them properly. If you have worked through all five and it still smells, send us the model name and what you have already tried and we will tell you what is left.