One of the things that I found in my 5 years as a Coast Guard Marine Inspector, was that small dry-chem extinguishers frequently have a common fault on board floating vessels, AND in automobiles. Simply put, the dry-chem powder inside the extinguisher "packs" to the bottom of the container due to vessel/vehicle vibration. This was very common with extinguishers that were approaching their end-of-life (10 years).
I got to where I could actually turn an extinguisher over, and FEEL whether the powder was loose enough that it would slide to the other end. Also, on occasion, when I was in a helpful mood, I would tap the body of the extinguisher with a rubber mallet and that would free-up the powder. After a couple more shakes, it would pass inspection.
I have thrown away a couple of extinguishers in my pickup truck after 7 or 8 years because I couldn't get the powder to move. When it gets to that state, it just becomes another boat anchor taking up space and providing false security.
Extinguishers are cheap. If you can't
hear, or
feel the powder move, then recycle it to a recharging facility, or else discharge it, and throw it away.
Safety First. 
--Bill B
PS. I only carry Halon in my Corvette.
