This study analyses NO_X emission deterioration in Euro V and Euro VI ABC diesel trucks using remote sensing data from Belgian and Swiss highways. Trucks are classified as rigid trucks or tractor trailers and articulated trucks with three or more axles. Euro VI ABC trucks comprise 80 % of the dataset, with 93 % of records representing tractor trailers or articulated trucks. A slight temperature dependence of NO_X emissions is noted, and for deterioration purposes, only data from ambient temperatures between 15 and 25 °C are used. NO_X emissions increase linearly with mileage, while the NO₂/NO_X ratio decreases following a polynomial trend. The NO_X deterioration rate for Euro VI ABC tractor trailers ranges from 0.09 to 0.15 ± 0.01 g/kg fuel per 100,000 km, corresponding to an overall increase of about 60–70 % over 700,000 km, with no substantial difference observed between Belgian and Swiss trucks. Most trucks remain within regulatory NO_X limits throughout their lifespan. However, a significant subset of high emitters is identified, with Euro V vehicles exceeding 20 g NO_X/kg fuel (∼4 g/kWh), and Euro VI vehicles exceeding 7 g NO_X/kg fuel (∼1.33 g/kWh). The share of high emitters among Euro VI ABC tractor trailers increases with mileage, averaging about 7–11 % at approximately 325,000 km across both Belgium and Switzerland. These vehicles increase NO_X emissions from the clean fleet when driving on highways by 30–50 %. The findings support updated deterioration functions and emission factors in the forthcoming Handbook of Emission Factors for Road Transport v5.1, aiding emission inventorying, regulation, and air quality management under Europe's Zero Pollution Ambition.