Dad’s short answer: It's one of three things: a roller that never fully dried, a dirty water tank that sat with debris in it, or a damp filter. Find which one by smelling each part individually — the source is usually obvious once you check.

Diagnose it in two minutes

Pull the roller out and smell it. A roller that's been sitting damp is the most common culprit by a wide margin, and it'll be obvious.

Then the dirty water tank. If it's been emptied but not rinsed, there's a film of food debris and hair residue inside it that's doing the smelling.

Then the filter. A filter that went back in damp, or one that hasn't been rinsed in a long time, holds moisture and grime.

Then the debris tray in the roller housing, which is where hair and solids collect and which people forget exists.

What actually fixes it

For a mildly sour roller: run a full hot-water self-clean, then a complete dry cycle, and don't interrupt it. If your machine has a weak dry cycle, take the roller out and let it air dry standing up.

For a roller that's genuinely gone: replace it. Once mildew is established in the fibers it doesn't come back out, and you'll be chasing the smell forever. Rollers are a consumable anyway.

For tanks and filters: wash thoroughly, and let everything dry completely before reassembling. Damp parts going back into a warm machine is how the cycle starts again.

And then change the routine. This problem is almost always a symptom of skipping the after-every-run rinse and dry, and it'll come straight back if the habit doesn't change.

Dad’s Take: A damp roller is the answer nine times out of ten. Let the dry cycle finish, every time, and this never happens.