Abstract: A process gas analysis system is provided. The system includes a probe insertable into a source of process gas and having a distal end and a chamber proximate the distal end. A gas sensor is mounted within the chamber and is configured to provide an electrical indication relative to a species of gas. A diffuser is mounted proximate the distal end of the probe and is configured to allow gas diffusion into the chamber. A source of calibration gas is operably coupled to the probe and is configured to supply calibration gas, having a known concentration of the gas species. Electronics are coupled to the sensor and configured to store a pre-calibration process gas concentration and to measure an amount of time (sensor return time) for the sensor response to return to the pre-calibration process gas concentration. The electronics are configured to compare a measured sensor return time with a known-good sensor return time to provide an indication relative to the diffuser.