from a rich to lean or from lean to rich by the engine controller until the sensor senses passage through the stoichiometric air fuel mixture when this happens the engine controller changes the direction of the ramping as a result the air fuel ratio hovers around stoichiometry at a so called limit cycle frequency fig2 c illustrates the induced emf versus time for the sensor 10 of the present invention as illustrated in fig1 as the air fuel ratio illustrated in fig2 a switches between rich and lean note the emf produced between the leads 24 26 is not driven by a power source rather it is induced by the sensor itself due to the difference created by the change in the concentration of oxygen when the air fuel ratio switches to either side of stoichiometry taking advantage of the transient nature of this phenomenon when the air fuel ratio has been lean for a time the output of the sensor 10 is small substantially zero since the steady state properties of the two electrode assemblies 15 17 are similar when the air fuel ratio changes through stoichiometry from lean to rich electrode assembly 15 senses the rich mixture arriving at the sensor location in the exhaust gas before electrode assembly 17 consequently for a short period electrode assembly 15 sees a rich air fuel mixture whereas electrode assembly 17 still sees a lean mixture under these conditions the sensor 10 acts like the conventional hego sensor and generates a high positive voltage 48 however shortly thereafter the second electrode assembly 17 also sees the rich exhaust gas again since the two electrode assemblies 15 17 have substantially similar steady state properties the voltage output of sensor 10 decreases.