European Union vehicle and engine regulations require measurement of non-volatile (solid) particles with diameter >23 nm at the dilution tunnel. In 2019 it was decided to include particles >10 nm in the post Euro 6/VI regulations. Recent studies showed that sub-23 nm measurements are not only susceptible to volatile artifacts (i.e. re-nucleation downstream of the evaporation tube of the Particle Number system) but also to non-volatile artifacts (i.e. non-volatile particles formed in the tubing between the vehicle and the particle number system or in the particle number system itself). In order to investigate the origin of the non-volatile particle formation, steady-state tests with a moped, a compressed natural gas (CNG) and a diesel vehicle while regenerating were conducted. Systems at the tailpipe and the dilution tunnel with evaporation tubes or catalytic strippers and condensation particle counters (CPCs) with 50% detection efficiencies at 2.5 nm, 4 nm, 10 nm, and 23 nm were used. The results showed higher concentrations of sub-23 nm particles at the dilution tunnel than at the tailpipe when the facility preconditioning was not appropriate, the exhaust gas temperature exceeded 300°C, and high concentrations of semi-volatile material were emitted (e.g. regenerations, lubricant oil).