Nitrogen oxides (NO_x) emissions from diesel vehicles are a serious environmental concern. Prior to the introduction of on-road tests at type approval, vehicle on-road NO_x emissions were found many times higher than the applicable limits. Retrofitting an existing vehicle is a short/mid-term solution. We evaluated a NO_x reduction retrofit system installed on a Euro 6b diesel passenger car both in the laboratory and on the road. The retrofit consisted of an under-floor SCR (selective catalytic reduction) for NO_x catalyst in combination with a solid ammonia-based dosing system as the NO_x reductant. The retrofit reduced NO_x emissions from 25% (50 mg/km) to 82% (725 mg/km) both in the laboratory and on the road. The minimum reduction was achieved at cold start cycles and the maximum at hot start cycles. The retrofit had small effect on CO₂ (fuel consumption). No ammonia emissions were detected and the N₂O increase was negligible at cold start cycles, but up to 18 mg/km at hot start cycles. The results showed that the retrofit technology could be beneficial even for high emitting Euro 6b diesel vehicles.