The CE stamping of the one hundred 1.3 GHz cryomodules for the European XFEL Linac is a main step in the process of the certification of the entire Linac as pressure equipment. Stringent requirements on materials and the quality of the welds of the pressurised components need to be satisfied to obtain the stamp. This paper summarizes these requirements, describes the process developed to qualify each module and summarises the rework campaign on the cavity helium vessels made necessary to obtain the required quality for a reliable and safe accelerator.