The answer to 'vacuum smells bad' is that a vacuum cleaner can develop an unpleasant odor for a few common reasons. This could be due to a buildup of dirt, hair, and debris in the vacuum's canister or bag, or from a dirty filter that needs to be cleaned or replaced. Additionally, if the vacuum has been used to pick up pet hair or other strong-smelling materials, those odors can linger in the machine.
To resolve a smelly vacuum, start by emptying the canister or replacing the bag if it's full. Then, check and clean or replace the filters as needed. You can also try running the vacuum with baking soda or a few drops of essential oil to help freshen the air. If the odor persists, it may be time to have the vacuum serviced by a professional cleaning technician.