Hi
I am not joking here. Discovered through trial and error.

Try and get a cold. (not a very severe cold) by having ice-creams/cold water:

Once you find you have a runny nose then:

In the morning, after you wake up, within first half hour, before you brush or eat/drink anything, You must drink 4 glasses (1 litre)of hot (not just luke warm) water. Do not eat/drink anything for the next 20 minutes. You must be able to notice that the white film at back of your tongue has vanished. Then brush your teeth & clean the back of your tongue. Have your breakfast & then again drink 2 glasses of warm water. Then after each meal it is important to have a glass of warm water.

I have discovered that when your cold is more intense, and if you do what I have said above, YOU WILL HAVE NO BAD BREATH EITHER THROUGH YOUR NOSE OR MOUTH.

Having cold is a small price to pay in order to keep bad breath at bay - at least till a cure is discovered.
I hope someone who is a researcher is also reading this, so he/she could get to the depth & find why & how bad breath primarily through the nose gets eliminated, when we have an intense cold combined by the drinking of hot water.Also please floss your teeth regularly.