Why does your bissell steam mop smell like you mopped with fish water when you use it?

You should purchase a new cartridge mine did the smae thing.

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Before buying a new cartridge, you might try cleaning the inside of the mop head (if possible with your model). It seems that the smell results from gunk and such building up inside the head unit.

I have a Bissell Lift-Off Steam Mop. The mop head is a two-piece plastic unit held together by eight screws. Remove the eight screws and gently pull the two pieces of the head unit apart. The steam hose is clamped into the bottom piece by way of a plastic bridge and two more screws. Remove the screws and bridge. Inspect the bottom piece of the mop head and for dirt and other stinky things. Clean thoroughly (along with the upper piece of the mop head still attached to the mop. Let it all dry thoroughly and then reassemble in the opposite manner as you took it apart. Not counting for drying time, the whole process took me about 5-minutes. After a few seconds of steam, there was no more smell.