get a new on, those gauges are wrong way too often for a piece of safety equipment

I ws trained in the use of extinguishers while in the Army. We used expired extinguishers from a maintenance company. They explained that they had to be maintained every 3 years, not because loss of pressure, but because seals and mechanisms could get stuck. Don't be in a sense of false security

As others said, replace it (service it if you can, doesn't hurt to have redundancy anyhow). Don't mess around with safety equipment, you want to be sure it works when you need it (and hope you never do), applies to everything from ppe to devices.