The change was adopted by the International Maritime Organization through Resolution MEPC.398(83) at MEPC 83 in April 2025. The resolution amends the NOx Technical Code 2008, the mandatory technical framework used alongside MARPOL Annex VI Regulation 13 to control NOx emissions from marine diesel engines.
In practical terms, the amendment addresses a problem that often arises only after a vessel is already in service. Conventional NOx certification is generally built around emissions testing of a “Parent Engine”, usually on a test bed, with comparable engines then certified as members of an Engine Family or Engine Group. That approach works relatively well when engines are being certified as part of a normal newbuilding or production programme. It is more difficult when an installed engine is later altered substantially or an owner decides that the engine should be certified to a different NOx Tier.
In those cases, there may simply be no suitable comparable engine available for conventional test-bed testing. MEPC.398(83) was designed to deal with that gap without lowering the underlying emissions-control standard.
A new route for testing modified engines
According to Lloyd’s Register, the amended Code allows greater flexibility in the Parent Engine testing process where an existing installed engine has undergone a substantial modification or where an owner or operator seeks additional certification to a NOx Tier that was not applicable to that engine at the time of installation.
The core concept is that testing may, in the relevant circumstances, be carried out on the installed engine itself. Once that engine has completed the approved testing process, a new Engine Group can be established. Other comparable engines that have undergone the same type of alteration may then be certified as Member Engines using the normal procedures, rather than each engine necessarily having to repeat a full emissions measurement programme.
That is a relatively technical change, but it has real project implications. Major engine modifications are often undertaken as part of a wider retrofit, conversion or emissions-compliance programme, and the regulatory certification route can become a critical path item if it is addressed only after the engineering work is complete.
Where the NOx rules sit within MARPOL Annex VI
The IMO’s NOx framework applies to many installed marine diesel engines above 130 kW, subject to specified exceptions, and uses different emissions “Tiers” depending broadly on the vessel’s construction date and, for certain Tier III requirements, the area in which the ship operates. Compliance is demonstrated through survey and certification, including the Engine International Air Pollution Prevention (EIAPP) Certificate and the engine’s approved technical documentation.
That means a major engine modification is not only a technical matter between an owner and a repair yard. It can affect statutory certification, onboard documentation and the evidence that must be available to demonstrate continued compliance with MARPOL Annex VI. The certification strategy therefore needs to sit inside the retrofit workstream from the beginning.
The September 2026 amendment does not replace the existing NOx regime or create a general exemption from testing. Instead, it provides a more workable route where the traditional Parent Engine model is difficult to apply because the engine is already installed and has subsequently been changed.
Why shipowners should address certification before the retrofit starts
For owners and ship managers, the most important practical point is timing. Lloyd’s Register notes that the amended process contemplates an Engine Emission Test Plan being prepared before testing begins. In a substantial retrofit, that can require early coordination between the owner, engine manufacturer, yard, recognised organisation and flag administration.
If the certification route is left until late in the project, the owner risks discovering that the testing configuration, instrumentation, documentation or approval pathway does not match what the administration or class requires. That can delay completion even where the engineering work itself has been finished.
The contract structure also matters. Retrofit and shipyard agreements commonly allocate responsibility for class approval, statutory certificates, tests and regulatory compliance, but those obligations are not always drafted with a change in certification methodology in mind. Owners should therefore check whether the yard or equipment supplier is responsible only for carrying out the physical modification or also for delivering the approvals and documentation necessary for the vessel to return to service.
Where delay damages or milestone payments depend on successful commissioning, a disagreement over who must arrange emissions testing can quickly become a commercial dispute rather than a purely technical issue.
The amendment arrives as the wider NOx regime continues to evolve
MEPC.398(83) is one part of a broader sequence of changes to the NOx Technical Code and MARPOL Annex VI. The IMO’s current amendments schedule shows further changes entering into force in 2027, including provisions dealing with multiple engine operational profiles and test cycles. Separately, IMO technical work has continued on how engine-testing methodology should deal with low- and zero-carbon fuels such as hydrogen and ammonia.
For fleet technical teams, that makes it increasingly important to treat NOx compliance as an evolving regulatory system rather than a one-off certificate obtained when the vessel is delivered. Engine replacement, major conversion, alternative-fuel projects and substantial modification can all reopen questions that may have appeared settled when the vessel first entered service.
What owners and managers should be checking now
Any owner planning a substantial engine modification should first determine whether the work triggers re-certification and whether the engine is being brought into a different NOx Tier. The next step is to agree the testing and approval pathway with the relevant recognised organisation and flag administration before the project reaches the testing stage.
Technical teams should also assemble the existing EIAPP documentation, engine technical file, prior test records and a clear description of the intended modification. Legal and commercial teams, meanwhile, should check whether shipyard, retrofit and equipment-supply contracts place responsibility for regulatory approval, repeat testing and delay on the correct party.
The amendment is designed to make certification more practicable for modified engines. It does not make the process automatic. Owners that integrate certification into the retrofit plan early are therefore likely to benefit most from the flexibility that the revised Code is intended to provide.
Sources
- International Maritime Organization, Amendments to IMO instruments: upcoming and recent entry into force/effective dates.
- IMO, Index of MEPC Resolutions and Guidelines related to MARPOL Annex VI.
- IMO, Nitrogen Oxides (NOx) – Regulation 13.
- IMO Resolution MEPC.177(58), NOx Technical Code 2008.
- Lloyd’s Register, “Engine test requirements for amended NOx Technical Code”, 3 December 2025.
Source note: Maritime Legal Business prepared this article from the official IMO materials and industry guidance identified above. Owners should confirm the position for the particular vessel, engine, flag and recognised organisation involved. This article is for general informational purposes and does not constitute legal advice.