New requirement for Selective Catalytic Reduction (SCR) Systems enters into force on 1 May 2026

Recent changes to the SCR Guidelines have altered how NOx measurement accuracy is assessed for catalyst monitoring. The former ±5% accuracy requirement has been replaced by a more flexible but less clearly defined concept of “sufficient accuracy”. This update places greater responsibility on applicants to justify the performance and monitoring of the lifetime of NOx catalysts.

Information for: Manufacturers, suppliers, and owners/managers.

Key changes to the requirements for NOx measurements

The requirements for NOx measurement devices used to monitor catalyst condition and potential degradation have been amended. Under the previous guidance, such devices were required to achieve an accuracy of ±5% relative to a reference NOx analyser (chemiluminescence detector, or CLD) compliant with the NTC 2008 standard. This explicit accuracy requirement has now been removed.

Under the 2025 SCR Guidelines, the NOx measurement method must instead be “sufficiently accurate” to enable the reliable monitoring of catalyst condition and degradation. However, the guidelines do not provide a specific definition or quantitative criterion for what constitutes “sufficiently accurate”.

Implications

Consequently, it is the responsibility of the applicant to demonstrate that the deviation between the on-board NOx measurement device and an NTC 2008‑compliant reference analyser remains within an appropriate range to support a dependable assessment of catalyst condition and degradation. In doing so, consideration may be given to factors such as the inherent measurement accuracy of the device, its expected measurement drift over time, and the margin to the applicable NOx limit value.

In addition, the assessment may need to address not only the raw NOx measurement results (e.g. concentrations expressed in parts per million), but also the evaluation process used to interpret these results and to determine whether the catalyst remains in an acceptable condition.

It should also be noted that NOx measurement accuracy is influenced not only by the performance device itself, but also by the exhaust gas sampling location and the degree of exhaust gas mixing at the sampling point. These aspects should therefore be duly addressed during testing and verification.

A proposal outlining recommended actions to support the implementation of the requirements under the 2025 Guidelines on SCR systems is provided in the Appendix to this news.

Recommendations

To ensure compliance with the 2025 SCR Guideline, it may be necessary to update the documentation, including the Technical Files, for individual engines or for existing engine groups or families.

In addition, the requirements of the 2025 SCR Guideline should be considered when planning and conducting parent engine emission tests and when preparing the associated documentation for new engine groups or families.

References

IMO resolution MEPC 399(83), 2025 Guidelines on SCR