One of my extinguishers was a disposable kitchen one, 8 years expired, had a test button on it. It performed flawlessly. The other one I used Was 2 years expired but the guage read full. It performed flawlessly. I have several other 'current' ones.

When I replaced the used ones, I asked about filling/testing/certification/etc., and was told that it costs less to have a full extinguisher certified, because you are not paying for the material fill. It actually wasn't a huge saving, but depending on the size of the extinguisher, it added up quick