Fire doors look simple — a door, basically — but they're engineered assemblies, and an inspection checks the whole assembly, not just whether it closes.
What the engineer actually checks
A proper inspection covers the door leaf itself for damage, warping, or unauthorised holes (anything drilled through for cabling, for example, can break the fire rating). They'll check the gaps around the door, which need to sit within a few millimetres on all sides for the door to actually hold back smoke. Intumescent seals and smoke seals get checked for damage or paint-over, since these are what expand in heat to seal the gap. Hinges, the door closer, and any hold-open devices all get tested to make sure the door closes fully and latches properly every time, not just most of the time. Glazing, if there is any, gets checked to confirm it's fire-rated glass rather than standard glazing that's been swapped in at some point.
How long it takes
For a single door, a thorough inspection usually takes a few minutes once the engineer's set up. For a whole building, it adds up — a site with twenty or thirty fire doors across multiple floors is a half-day job, sometimes more if access to some doors is awkward or they're locked when the engineer arrives.
What counts as a fail
Common fail points include gaps that are too wide (often from a door warping over time or being rehung incorrectly), damaged or missing seals, a door that doesn't self-close properly, or signage that's missing or wrong. None of these are usually a same-day emergency, but they do need fixing on a reasonable timescale and recording in your logbook either way.
How often it needs doing
Frequency depends on the building. High-traffic communal doors in larger or higher-occupancy buildings typically need checking more often than a door inside a small single-tenant office. Either way, it's not a one-off — doors get knocked, hinges loosen, and seals degrade over time, so this sits on the same kind of recurring schedule as your extinguishers and alarms.