This study investigates the failure mechanisms of common-rail fuel injectors used in state- of-the-art dual-fuel marine engines. Combining numerical simulations and experimental tests, the research analyses the operational dynamics of the injector used in the diesel-mode cylinders that provide the required boost pressure for the dedicated reactivity-controlled compression ignition (RCCI) cylinder. A 3D computational fluid dynamics (CFD) model was developed to evaluate temperature and fuel velocity fields within the injector, while engine tests identified critical failure modes linked to abnormal fuel injection behaviour. Key findings reveal that a fuel supply limitation in the enginés speed control idle mode triggers uncontrolled speed drops when loads exceed 267 kW, leading to secondary injection peaks and prolonged high-temperature exposure. Thermal simulations highlight significant non-uniform temperature distributions, with the injector’s bottom region reaching 435 °C during high duration operation, doubling low-state values. This elevated temperature propagates to the injectoŕs solenoid coil and causing insulation degradation. The study concludes that sustained high temperature, exacerbated by boundary condition shifts, is a primary driver of coil insulation failure. These insights emphasise the need for enhanced thermal management strategies and adaptive control algorithms to mitigate injector failures in marine and high-efficiency engines.