Reference Air Path: Designed with a channel (usually a small hole/slit) internally within the probe or between the probe and the protective sleeve, allowing oxygen from ambient air to naturally diffuse to the inner electrode, providing a reference gas with a constant oxygen partial pressure (~20.95% O2). This entry point is typically located at the probe's rear end or near a clean area adjacent to the flange. Key Point: Ensure the reference gas port remains unobstructed to prevent failure of the probe.