Safety first: Use appropriate cleaning agents and ensure good ventilation.

Run a maintenance wash: Run an empty 90°C cycle with a washing machine cleaner or white vinegar and baking soda to kill bacteria and remove residue.
Clean door gasket: Wipe the gasket folds with a cloth and mild detergent or diluted bleach. Dry thoroughly.
Clean detergent drawer: Remove the detergent drawer and scrub away mold and residue. Clean the drawer housing as well.
Leave door open: After each use, leave the door and detergent drawer slightly open to allow drying and prevent mold growth.

Washer-dryer combo has a persistent musty or moldy odor inside the drum.

Possible Causes