Method, device and service quality problem of detection system

The invention relates to a method for locating a QoS problem implemented on a terminal during a multimedia session established between two terminals through one or more communications networks comprising at least one media gateway adapted to retransmit media data packets without special treatment in a first management says "router mode", the terminal obtaining a first quality of service data (20) on the multimedia session. The process is such that it comprises, following a change of management mode of the media session by the media gateway to a management method called "relay mode" in which the multimedia session established between the terminal initially split into two sessions different media, a step of obtaining (23) a second data quality of service relating to one of the two media sessions established following the change in the media gateway management mode, a comparison step (24) of the first and the second QoS data and a locating step (25) of the service quality problem depending on the result of the comparison.