Reference Air Path: A passage (typically a small hole or gap) is provided within the probe or between the probe and the protective sleeve, allowing oxygen from the ambient air to naturally diffuse to the inner electrode, providing a reference gas with a constant oxygen partial pressure (~20.95% O2). The reference gas inlet is usually located at the probe's rear end or near the flange in a relatively clean area. The key point is: it is essential to ensure that the reference gas passage remains unobstructed; any blockage will render the probe inoperable.