© IMechE 2022.The typical negative effects of prolonged slow steaming operations are investigated in this study. The scope of the research is to examine the effects of the carbon deposit formation on piston rings (lack of sealing function), exhaust boiler (reduction in the heat recovery capacity), turbocharger (lack of scavenging capacity), and injector, due to the prolonged slow steaming operation. It is necessary to identify the holistic adverse effects of the low-load operation on the main engine performance and subsequent components. The study shows that the negative consequences of a long-term slow steaming operation cause noteworthy efficiency degradation in marine diesel engines. The paper aims to clarify the barriers to the efficient operation of marine diesel engines via raising awareness of proper and planned maintenance for sustainable slow steaming. The degradation rates affect the total operational efficiency, CO2 emissions, and fuel consumption. The study results show that the fuel consumption increases by 1.9%, 2.1%, and 1.9% of daily consumption and the corresponding CO2 emission increments are 4.36, 4.29, and 3.48 kg CO2 per nautical mile sailing of the container ship at specified speeds at 65%, 55%, and 45% engine loads, respectively. The efficiency variation leads significant amount of emission increment, while up to 50% decrement will enter into force by April 2022 for container ships. The study gives valuable insight into the increase in CO2 emissions and fuel after long-term slow steaming for the near future with the stricter emission limits. The results provide considerable information about the deterioration effect on the whole energy system and help to estimate potential efficiency levels for marine diesel engines.