What are you puzzled about?
It sounds like he's got you second guessing what he knows is a lie...and you want to believe him so badly you're going to look for possibilities that the breathalyzer is defective.
You know it's a lie. What more do you need to know.
Just curious as to why you are policing him with a breathalyzer machine anyway? If this is something you feel you need to do with a boyfriend, a significant other is keep them under lock and key....you need to reconsider this relationship.