A dry chemical extinguisher with a gauge lets you know if it still has pressure, but that can give you a false sense of security.need to have the dry chemical recharged every 6 years (Transport Canada).
Unfortunately most of the dry chemical units have a design that allows the powder to pack together over time, due to vibration.
You can go to spray and much of the powder stays inside as a mass.
Definitely need to be inspected and recharged, regardless of regulations.
Ages ago Ansul came up with a design with a removable CO2 pressurizing cartridge with a tube that goes down to the bottom of the vessel with a spyder on the bottom.
Normally the main vessel is at atmospheric pressure.
Push the button and it punctures the cartridge, which then blasts CO2 into the vessel and simultaneously loosens the dry chemical and pressurizes the system.
They can sit in vehicles for ages and still work. All you need to check is the weight of the CO2 cartridge.
Unfortunately the regulations apply to all dry chemical units, even ones that will work after sitting for ages.
But.. most dry chemical units can pack hard with vibration and not work when needed.
Not sure about CO2 extinguisher regulations now.
Technically.. checking the weight is enough, but after enough years, they need to be replaced or inspected and pressure tested. (Just like propane tanks)