Def is 32.5% ammonia and the rest is distilled water. There better not be any thing else in there. Any more ammonia and you will get what is called ammonia slip past the scr and the rear nox will see it as a rise in outlet nox. Any less and the scr won’t convertSome companies have been adding additives lately. They can somehow interfere with the system and lead to a DEF Quality Poorq warning. I don't know why it happens.