Exhaust gas sensor for determining A/F ratio

An oxygen sensor for detecting a measurement gas existing in an external space, which has a sensing element comprising:

• means for defining a gas-diffusion space which communicates with said external space, said gas-diffusion space having a predetermined resistance to diffusion of the measurement gas therethrough, said gas-diffusion space including a first portion to which the measurement gas diffuses along a first diffusion path, and a second portion to which the measurement gas diffuses through said first portion along a second diffusion path which has a larger length than said first diffusion path;
• first oxygen sensing means for producing a first output representative of an oxygen concentration of an atmosphere in said first portion of said gas-diffusion space;
• first oxygen pumping means responsive to said first output from said first oxygen sensing means, for effecting an oxygen pumping action for controlling the oxygen concentration of said atmosphere in said first portion of said gas-diffusion space;
• second oxygen sensing means for producing a second output representative of an oxygen concentration of an atmosphere in said second portion of said gas-diffusion space;
• and
• second oxygen pumping means responsive to said second output from said second oxygen sensing means, for effecting an oxygen pumping action for controlling the oxygen concentration of said atmosphere in said second portion of said gas-diffusion space.