Dad’s short answer: Almost always one of three things: a dirty tank that was emptied but never rinsed, residue left in the hose, or a brush put away wet with hair on it. All three come from the same cause — skipping the rinse after a job.

Find it in two minutes

Smell the dirty tank first. If it was emptied but not rinsed, there's a film of whatever you pulled out of the carpet sitting inside it, and in a warm closet that goes sour fast. This is the most common answer by a wide margin.

Then the hose and tools. Anything that went through a pet mess needs flushing with clean water, not just draining. Residue in a hose is out of sight and very much not out of mind a week later.

Then the brush roll. Wet, with hair wrapped on it, put away in a closet — that's a smell source and it's also how brushes go bad.

And check any filter the machine has. Small filters on pet tools clog and hold residue.

The routine that prevents it

Empty and rinse the dirty tank after every single use. Not just empty — rinse.

Run a tank of clean warm water through the machine and the hose at the end of a job, so what's in the lines gets flushed out.

Pull the brush, clear the hair, rinse the tools, and leave everything to air dry before it goes back in the closet. Tanks stored with the caps off dry far better.

That's about three minutes, and it's the whole difference between a machine you'll own for years and one that becomes unpleasant to use by month four.

If it's already bad, a rinse cycle with warm water and a small amount of white vinegar through the machine usually clears it, followed by a plain water rinse.

Dad’s Take: Rinse the dirty tank every single time and leave everything to air dry. That one habit prevents almost every complaint in this category.