Extinguishers feel like the kind of thing you hang on a wall and forget about. They're not. Most premises run on two overlapping service cycles, and missing either one is a common reason businesses fail an inspection.
The annual basic service
Every extinguisher needs a basic service once a year, carried out by a competent engineer. This covers a visual check of the casing for damage or corrosion, confirming the pressure gauge is in the right zone, checking the pin and tamper seal haven't been disturbed, and weighing certain types to confirm they haven't lost charge. It's quick per extinguisher, but it's also the check most likely to get missed simply because nothing about the extinguisher looks obviously wrong from the outside.
The five-year extended service
On top of the annual service, most extinguisher types need a more thorough extended service every five years. This typically involves discharging the extinguisher to check it actually works under pressure, then refilling and re-pressurising it. It's a more involved job, and it's the one businesses are most likely to be overdue on simply because five years is long enough to lose track.
Discharge testing
Separate from routine servicing, a sample of extinguishers may need full discharge testing at set intervals depending on type, to confirm the extinguishing agent itself still performs as expected. This is less about the hardware and more about whether what's inside the canister still does its job.
What happens if you skip it
An extinguisher that hasn't been serviced isn't necessarily useless, but you have no way of knowing that without the check, which defeats the purpose of having it. It also tends to be one of the first things a fire and rescue inspector or insurance assessor checks, since service tags with dates are a quick, visible way to judge how seriously a premises takes the rest of its compliance.