A method for locating faults in a passive optical network, PON, wherein said method comprises the following steps: providing a number of passive unique optical correlators at a number of respective diagnostic points of the PON (PC1, PC2, PC3, PC4), each of the passive unique optical correlators being configured to respond to receipt of a pattern (PATTERN1, PATTERN2, PATTERN3, PATTERN4) representing a respective signature and being responsive to an optical diagnostic signal by returning a predetermined unique matching signal; transmitting the diagnostic optical signal in said PON towards said diagnostic points, wherein said diagnostic signal comprises a plurality of respectively different signal patterns (PATTERN1, PATTERN2, PATTERN3, PATTERN4) stored in a data base, each representing a respective unique signature and associated with a respective passive unique optical correlator (PC1, PC2, PC3, PC4), and being the respective unique signature of each of the passive unique optical correlators different from the respective unique signature of any other one of said passive unique optical correlators, thereby enabling generation and return of a predetermined unique matching signal (M/CODE 1, M/CODE 2, M/CODE 3, M/CODE 4) by a specific passive unique optical correlator in response to receiving, at said specific passive unique optical correlator, the diagnostic signal that comprises the respective signal pattern (PATTERN1, PATTERN2, PATTERN3, PATTERN4); receiving upstream response signals from one or more of said plurality of passive unique optical correlators (PC1, PC2, PC3, PC4) which had received the diagnostic signal; detecting a presence or an absence, in said received upstream response signals, of a predetermined unique matching signal expected from a specific passive unique optical correlator out of said passive unique optical correlators; and judging about a fault in proximity of a specific diagnostic point, based on the presence or absence and/or a level of said predetermined unique matching signal (M/CODE 1, M/CODE 2, M/CODE 3, M/CODE 4) expected from the specific passive unique optical correlator (PC1, PC2, PC3, PC4) placed in said specific diagnostic point. providing a number of passive unique optical correlators at a number of respective diagnostic points of the PON (PC1, PC2, PC3, PC4), each of the passive unique optical correlators being configured to respond to receipt of a pattern (PATTERN1, PATTERN2, PATTERN3, PATTERN4) representing a respective signature and being responsive to an optical diagnostic signal by returning a predetermined unique matching signal; transmitting the diagnostic optical signal in said PON towards said diagnostic points, wherein said diagnostic signal comprises a plurality of respectively different signal patterns (PATTERN1, PATTERN2, PATTERN3, PATTERN4) stored in a data base, each representing a respective unique signature and associated with a respective passive unique optical correlator (PC1, PC2, PC3, PC4), and being the respective unique signature of each of the passive unique optical correlators different from the respective unique signature of any other one of said passive unique optical correlators, thereby enabling generation and return of a predetermined unique matching signal (M/CODE 1, M/CODE 2, M/CODE 3, M/CODE 4) by a specific passive unique optical correlator in response to receiving, at said specific passive unique optical correlator, the diagnostic signal that comprises the respective signal pattern (PATTERN1, PATTERN2, PATTERN3, PATTERN4); receiving upstream response signals from one or more of said plurality of passive unique optical correlators (PC1, PC2, PC3, PC4) which had received the diagnostic signal; detecting a presence or an absence, in said received upstream response signals, of a predetermined unique matching signal expected from a specific passive unique optical correlator out of said passive unique optical correlators; and judging about a fault in proximity of a specific diagnostic point, based on the presence or absence and/or a level of said predetermined unique matching signal (M/CODE 1, M/CODE 2, M/CODE 3, M/CODE 4) expected from the specific passive unique optical correlator (PC1, PC2, PC3, PC4) placed in said specific diagnostic point.