Less nitrogen oxides are produced and this means more free oxygen. Gasoline is predominately carbon and hydrogen. The exhaust products are a mixture of carbon monoxide, carbon dioxide, water, unburned hydrocarbon molecules, oxygen, nitrogen and oxides of nitrogen. As well as all the other nasty additives that are included in fuel these days. The catalytic converter is used to complete the process and thus reduce pollution.
Additional air is added prior to the converter to enable it to function.
How does the oxygen sensor work Nearly all automotive sensors are of the zirconia type. A ceramic bulb projects into the hot exhaust gas. The bulb is hollow and the inside surface is in contact with the atmosphere outside the exhaust pipe. Both surfaces are coated with platinum. Oxygen atoms are negatively charged and will attach to the platinum surfaces therefore building up a negative charge on each surface.