Abstract: Currently， gear fault detection based on vibration， acoustic emission， and other signals requires the installation of additional sensors. This approach faces limitations in terms of sensor placement， high sensor cost， and difficulties in analyzing signals due to modulation effects in gear systems with variable transmission paths， such as planetary gearboxes. A method for diagnosing local gear faults using the built-in encoder of the servo motor is proposed in this paper. Using the output signal of the built-in encoder to extract the feature related to local gear faults. The encoder signal acquisition wiring is drawn from the built-in encoder of the servo motor， and a high-speed counter is used to record the time interval between the rising edges of the angular position pulses of the rotary encoder. The instantaneous angular speed （IAS） signal is calculated， and the IAS signal is analyzed and feature extracted in the angular and order domains to achieve local gear fault detection. Taking the planetary gearbox gear localized fault detection as an example， the proposed method is validated through experiments. Results show that using the built-in encoder of the servo motor can effectively achieve the detection of gear partial faults under low and variable speed conditions. This provides a new approach to fault detection of transmission units such as gearboxes in applications driven by servo motors.