Long term compliance with emission standards is essential for reducing the environmental and health impacts of diesel trucks. However, nitrogen oxide (NOₓ) emissions increase over time. This study analyses NOₓ emission deterioration in Euro V and Euro VI ABC trucks, using several thousand remote sensing records collected on highways in Belgium and Switzerland. Vehicles were grouped into two size classes—rigid trucks and tractor trailers—with Euro VI ABC trucks comprising approximately 80% of the dataset. About 93% of the records correspond to tractor trailers, making them the dominant vehicle type in the analysis. Given the observed influence of temperature on NOx emissions, the analysis focused on records within the range of 15–25 °C. Linear regression was used to estimate NOₓ increases with mileage, while polynomial regression assessed the decline in the NO₂/NOₓ ratio, indicating after-treatment system degradation. High-emitting vehicles were identified based on emission records exceeding thresholds derived from portable emission measurement system (PEMS) trips and simulations: 20 g NOₓ/kg fuel for Euro V vehicles and 7 g NOₓ/kg fuel for Euro VI ABC vehicles. These vehicles significantly increase fleet average emissions and the apparent rate of deterioration. In Belgium, the share of high emitters among Euro VI ABC tractor trailers increases from 5% at 50,000–100,000 km to approximately 11% at 300,000–350,000 km. In Switzerland, the share increases from 2% at 50,000–100,000 km to around 9% at 400,000–450,000 km. The rate of deterioration for Euro VI ABC tractor-trailers was 0.09 ± 0.01 g NOₓ/kg fuel per 100,000 km in Switzerland and 0.15 ± 0.01 g NOₓ/kg fuel per 100,000 km in Belgium. Since mileage data are sourced from external databases rather than directly measured, the robustness of the country-specific results was tested using Monte Carlo simulations (±25% uncertainty on mileage data), showing minor variability. The weighted average rate of deterioration, accounting for differences in record counts and emission levels between countries, was 0.13 g NOₓ/kg fuel per 100,000 km. However, when high emitters were included, the fleet-level rate of deterioration increased to 0.30 g NOₓ/kg fuel per 100,000 km in Switzerland and 0.50 g NOₓ/kg fuel per 100,000 km in Belgium. The weighted average rate of deterioration similarly increased to 0.44 g NOₓ/kg fuel per 100,000 km, reflecting nearly a threefold rise. Emissions over the useful life of the vehicles were compared with regulatory limits, and after excluding high emitters, NOₓ emissions were found to be below the limits. This study provides one of the few large-scale, real-world assessments of NOₓ deterioration, incorporating ambient temperature effects, vehicle size classes, high emitter influence, cross-country weighting, and statistical validation. It offers valuable insights for emissions regulation and policy design, and its findings are used for the deterioration functions in the forthcoming Handbook Emission Factors for Road Transport (HBEFA) v5.1.