Many causes/sites. Bad breath can have GI cause, as simple as what food's eaten. Bowel blockage, lesions can cause odor. A physician can evaluate. If a tooth is decaying, it might have a persistent odor. That decay can occur even under a crown. A patient should keep asking the dentist or doctor to evaluate if no cause of persistent smell is found. Dental/tongue hygiene is important no matter what cause. See comments.
Self diagnose it! Other than common food odors that come from ingesting garlic etc. Entering the blood and exit the lungs (or rare stomach malignancies) most bad breath comes from the surface of the tongue, ;/or from between the teeth. Floss twice daily. (skipping one day is like not flossing at all). And brush your tongue, with a touch of toothpaste, the farther back the better. Odds on, bad breath reduced!
Periodontal. Possibly periodontal disease. Could be infected tonsils. Post nasal drip. A broken rotting tooth. And many more possibilities. Start with a visit to a dentist to see what your oral condition is if it has been a while. Other possibilities are GI related, but it would seem best to rule out your dental, gum and oral health first.
Bacteria... ... and the toxins being released. Cavities, periodontal disease, even rotting trapped food. See a local dentist for an evaluation.
Floss,scrape,reflux. Could all be reasons for a foul dental odor. First you have to look at your dental hygiene. Do you brush and floss regularly? If you don't, floss, bacteria gets lodged deep in your gums and your teeth and rots, so to speak. Do you have a white coat of film on your tongue.. Bacteria heaven. Get a tongue scraper, floss, brush, scrape and rinse with Any mouth rinse that has chlorine dioxide (breathRX.
Mouth Smell. Three causes of mouth smell. Naso-pharynx.(sinuses), mouth(dental) or gi. That's it. Can be a low grade to a high grade infection. Get it checked out. Dental is the most common, but get it checked out. Each area has it own specialist that can help you a lot. Get it checked out. Best.
Can be due to a host. Of reasons - limited details offered in question; could be due to -- poor oral hygiene, gum disease; food impacted between teeth, medication effects; and the list goes on. First step is to consult with a dentist asap for a comprehensive exam, x-rays and cleaning. In the interim, invest in a good mechanical toothbrush (ex: oral b triumph) and a waterpik and use them daily. Hope this helps.
Dental Abscess. From a dental standpoint, bad breath is directly related to bacteria in the mouth and often infection. Bacteria can stay in your mouth for extended periods of time in the form of plaque which turns into the hard deposits we refer to as calculus or tartar. You need to have tartar removed by a dental hygienist. When associated with a pimple like infection near a decaying tooth, it may be an abscess.
Clean better. The tongue collects quite a bit of food, i believe the french used to use a toungue scraper like a butter knife to scrape across the tongue and remove food from the crypts of the tongue. Brushing your tongue with the tooth brush will work too. Tonsils can also have crypts with a lot of decaying food, have your doctor look for these.