Ultrasonic Flow Meters (USMs) are essential instruments for accurate and reliable natural gas measurement in custody transfer and industrial applications. However, their performance can be significantly compromised by the accumulation of debris particles within the meter spool, sensor paths, and transducer chambers. These contaminants disrupt the ultrasonic signal transmission, distort the flow velocity profile, and introduce substantial errors in volumetric calculations. This study focuses on the RMG GT400 ultrasonic flow meter deployed at PT Pertamina Gas Operation East Java Area, which exhibited a measurable decline in performance attributable to internal contamination. Diagnostic analysis prior to maintenance revealed sensor path degradation, with some paths operating at as low as 16% effectiveness. Following a comprehensive internal cleaning of the meter spool and sensor chambers, post-maintenance diagnostics indicated a complete restoration of signal integrity and measurement accuracy, with all sensor paths returning to optimal 100% functionality. The case study demonstrates that regular inline diagnostics combined with timely mechanical cleaning can effectively mitigate the adverse effects of debris, preserve flow profile fidelity, and ensure long-term metering accuracy. Additionally, the utilization of performance indicators such as Gain, Signal-to-Noise Ratio (SNR), and Path Velocity Symmetry is highlighted as a strategic approach to detecting early signs of performance degradation. These findings underscore the importance of integrating predictive maintenance protocols and continuous performance monitoring into gas metering operations.