Underlying cause? Halitosis (bad breath) has many causes, often not as obvious as onion salad or needing a breath mint / chewing gum or to brush your teeth. Gi problems (e.g. Reflux), sinus infection, metabolic issues like liver disease, and oral/throat cancer (usually obvious) can all cause bad breath. You might start with your dentist, chewing gum, and go from there!
Mouth Odor/Halitosis. Health conditions like kidney and liver disease, diabetes, tonsil stones, chronic lung and sinus infections as well as poor dental hygiene can cause mouth odor. Certain medications and foods also can contribute. Try flossing & brushing twice daily, gargle with hydrogen peroxide 2x/day or use chlorohexidine as advised by your dentist. Avoid causal foods and work w/ your pmd for health maintenance.