The present invention relates to a machine (10) for sterilizing plastic preforms for the manufacture of containers, characterized in that the machine (10) comprises a device (50) for detecting the fouling of at least one sterilizing-fluid conveying line (38) extending fully or partially through an injection nozzle (30), said detection device (50) comprising at least emitter means (52), receiver means (56) and a control unit (70) for determining the degree of fouling of said conveying line (38) by comparing the intensity of the light radiation emitted at one (40, 48) of the ends thereof with that received at the other (40, 48) of the ends thereof.