The invention relates to a system for the certification of a passive optical network and for the detection of problems and faults in the last fibre legs, which is suitable for use in passive optical networks which, between the OLT device housed in a telephone central office and the user device of ONT, includes optical dividers or splitters which, in the downward direction, distribute the optical signal to all of the users within a tree-and-branch architecture, and in the upward direction, combine the optical signals from all of the ONTs. The system is based on the incorporation of a group of devices known as an OLT emulator and a remote testing unit which are joined to the corresponding wall socket of the telephone central office and user, such that the remote testing unit is positioned facing an OLT in operation, allowing the certification of the last leg of the optical network and the detection of problems and faults in a PON network in operation.