The answer to 'air duct cleaning smells bad' is that a bad odor after duct cleaning is often caused by lingering dust, debris, or moisture left behind in the ductwork. This can lead to the growth of mold, mildew, or bacteria that emit unpleasant smells. Other common causes include using low-quality cleaning methods or products, or disturbing pests or dead animals in the ducts.
To resolve the issue, you'll need to have the ducts thoroughly cleaned and sanitized by a professional HVAC cleaning service. They have the specialized equipment and techniques to properly remove all contaminants and ensure the ducts are left clean and dry. Attempting to DIY duct cleaning is not recommended, as you risk further spreading debris and odors throughout your home.