I thought my vacuum was dying. It was making a sad wheezing noise and leaving behind more crumbs than it picked up. I was two days from buying a new one — a three-hundred-dollar impulse — when a YouTube comment saved me. “Have you changed the filter?”

I had not. I did not even know my vacuum had a filter. I had owned it for four years.

The filter nobody tells you about

Most vacuums have at least two filters: a pre-motor filter that protects the motor from debris, and a post-motor filter (often a HEPA filter) that cleans the air before it blows back into your room. Some have a third — a foam filter wrapped around the motor housing.

These things clog. Slowly. So slowly you do not notice it happening. One day your vacuum runs at full power. Three months later it is pushing dust around instead of sucking it up. You blame the vacuum. It is not the vacuum.

How often you should actually change them

• Pre-motor filter: Rinse it every month. Tap the dry dust out every week. Replace it every six months.
• HEPA filter: Replace every six to twelve months depending on how much you vacuum. Pet owners: closer to six.
• Foam filter: Rinse and squeeze dry every two months. Replace when it starts looking ragged.

I now have a recurring phone reminder. First of every month: clean the filters. It takes five minutes and costs nothing.

The signs you waited too long

A clogged filter strains the motor. The vacuum gets louder and hotter. Suction drops. Fine dust starts blowing out the exhaust port instead of getting trapped. You might smell something like burning — that is the motor overheating.

If your vacuum has lost suction and you have emptied the bin and cut the hair off the brush roll, clean the filters. It fixes the problem about eighty percent of the time.

Where to find replacement filters

Do not go straight to the manufacturer’s website and pay thirty dollars for what is essentially a folded piece of paper. Amazon and eBay sell third-party compatible filters for a fraction of the price. Search your vacuum model number plus “HEPA filter replacement.” Read a couple reviews to make sure the fit is right, but these are simple parts — hard to mess up.

One warning: washable HEPA filters say “washable” right on them. If yours does not say washable, do not wash it. Water ruins the paper medium. Tap it clean and replace it.

I saved three hundred dollars with a twelve-dollar filter and five minutes of work. I wish I had known sooner.