We propose and demonstrate a new in-service fault-localization method for a wavelength division multiplexing passive optical network (WDM-PON). This scheme uses a tunable OTDR realized by a wavelength-locked Fabry-Perot laser diode. We successfully detect the faults both at the feeder fiber and the drop fibers. The resolution and the dynamic range are 100 m and 12 dB, respectively. In addition, the crosstalk induced by the OTDR signal to the transmission data is negligible.