How to stop bad smell coming out of SEVITE3 pump under the sink? ?

Just run bleach into the sink and fill with water pull the plug and put some bleach down the sink overflow.

Turn the electric off to the box and let it steep in the bleach mix for a while then turn it on to flush it clean,
make sure when doing this nothing comes out of the air vent and remove the charcoal filter if fitted.

Do this regularly and the smell will go away.

The machine is basically a tank and as it's used a tide mark of floating matter builds on the sides of the box building up on the sides and coating it this needs cleaning away, if the box is new then the smell will be from the associated pipe work and especially the overflo

Muncher
April 2015

The smell will come from bugs living in the pump - or in the pipework connected to it or perhaps even the drains - probably not the drains if there's an S bend in the system.

Hot Water
or
Vinegar
or
Bleach

Flush through - and repeat overnight - flush again

Regular cleaning will obviously help...
March 2015