A detainable deficiency is a finding serious enough that the ship should not go to sea: essential equipment the crew cannot operate, a required certificate that is invalid, or a clear hazard to safety, health or the environment. The PSC officer records it with action code 30, and a single code 30 finding is enough to detain the ship. It is the line between an inspection with findings and a detention.
The test the officer applies is professional judgement against convention requirements, guided by the MoU's published criteria: is the ship fit to proceed to sea without danger to the ship or the people on board, and without unreasonable threat to the marine environment? If not, the deficiency is detainable.
How it shows up in practice
Most deficiencies never come close to the bar. A missing light bulb in a navigation light gets a rectify-before-departure code; an expired chart correction gets a deadline. What crosses the line is failure of something the ship depends on. A lifeboat engine that will not start on demand. An oily water separator with signs of bypassing. Hours of rest records that collapse under comparison with the port log, suggesting the watchkeepers are exhausted. Crew unable to run an emergency generator drill when asked. Each of these has detained ships, and several of them are as much about the crew's familiarity as the hardware. The action code attached to each finding, not the raw deficiency count, tells you how serious an inspection really was; the code system is laid out in PSC deficiency codes explained.
What it means for operators
Once a code 30 is recorded, the ship is detained until the deficiency is rectified and verified, and the detention becomes a permanent record with consequences well beyond the port stay, detailed in what a detention costs. The practical lesson sits earlier in the timeline. Nearly every detainable deficiency is findable in advance by the crew: pumps can be run, records can be cross-checked, drills can be rehearsed. A pre-inspection that actually operates the equipment, with photographic evidence reviewed ashore, is the cheapest insurance available, and it is what Fleetward's PSC workflow turns into a routine rather than a scramble.