(11)in Article 17a, the following paragraphs are added:‘3.With effect from 1 January 2021, national authorities shall refuse, on grounds relating to emissions, to grant EU type-approval or national type-approval in respect of new types of vehicle or engine which do not comply with the requirements of this Regulation as amended by Commission Regulation (EU) 2019/1939note.By way of derogation from the first subparagraph, new types of positive-ignition engines, type 1A dual-fuel engines and type 1B dual-fuel engines (in dual-fuel mode), and vehicles equipped with such engines, shall comply with the maximum allowed conformity factor for PM number according to point 6.3 of Annex II with effect from 1 January 2023. However, as from 1 January 2021, the particle number work window conformity factor and CO2 mass window conformity factor shall be stated in the PEMS demonstration test results on the type-approval certificate for monitoring purposes.4.With effect from 1 January 2022, national authorities shall, in the case of new vehicles which do not comply with the requirements of this Regulation as amended by Regulation (EU) 2019/1939, consider certificates of conformity issued in respect of those vehicles to be no longer valid for the purposes of Article 48 of Regulation (EU) 2018/858 and shall, on grounds relating to emissions, prohibit the registration, making available on the market and entry into service of such vehicles.By way of derogation from the first subparagraph, with effect from 1 January 2024, national authorities shall, in the case of new vehicles equipped with positive-ignition engines, type 1A dual-fuel engines and type 1B dual-fuel engines (in dual-fuel mode) which do not comply with the maximum allowed conformity factor for PM number according to point 6.3 of Annex II and the requirements of this Regulation as amended by Regulation (EU) 2019/1939, consider certificates of conformity issued in respect of those vehicles to be no longer valid for the purposes of Article 48 of Regulation (EU) 2018/858 and shall, on grounds relating to emissions, prohibit the registration, making available on the market and entry into service of such vehicles. However, as from 1 January 2022, the particle number work window conformity factor and CO2 mass window conformity factor shall be stated in the PEMS demonstration test results on the type-approval certificate for monitoring purposes.With effect from 1 January 2022 and except in the case of replacement engines for in-service vehicles, national authorities shall on grounds relating to emissions prohibit the making available on the market and entry into service of new engines which do not comply with the requirements of this Regulation as amended by Regulation (EU) 2019/1939.By way of derogation from the third subparagraph, with effect from 1 January 2024, and except in the case of replacement engines for in-service vehicles, national authorities shall, on grounds relating to emissions, prohibit the making available on the market and entry into service of new positive-ignition engines, new type 1A dual-fuel engines and new type 1B dual-fuel engines (in dual-fuel mode) which do not comply with the requirements of this Regulation as amended by Regulation (EU) 2019/1939.