We propose a method to locate precisely faults in wavelength-division-multiplexing (WDM) passive optical network (PON) by using a wavelength tunable chaotic laser. The chaotic laser consists of a multiple-longitudinal-mode Fabry-Perot (FP) laser diode whose modes match the channels of WDM-PON, and an optical feedback loop including a filter. The loop feeds a proportion of light of one mode that passes through the filter back into laser cavity to generate chaotic light. By adjusting the filter frequency, we can tune the wavelength of the chaotic light, and diagnose the corresponding branch of WDM-PON. We demonstrate a proof-of-concept experiment for detection of three ITU channels. Fault location is realized by correlating the back-reflected light with its time-delayed duplicate. The results show that spatial resolution of 2 cm and dynamic range of about 20.8dB can be achieved. In addition, we have experimentally studied the effects of the strength level and wavelength mismatching of the feedback light on the chaotic output of the FP laser.