The oxygen sensor, also called a lambda sensor, is mounted in the exhaust manifold, or the engine pipe. Its sensing portion is exposed to the stream of exhaust gas. It detects left-over oxygen in the exhaust gas, and sends the data to the control unit.
The control unit uses it to fine-tune the pulse it sends to the fuel injectors.
The sensor consists of a tube, closed at one end, and made of Zirconia ceramic, or Titanium ceramic. Its inner and outer surfaces are coated with platinum. The outer closed end is covered by a louvred metal shroud that protects it from breakage but still lets the exhaust gas contact the tube. Its inner surface is in contact with the air. A wire contacts the inner surface of the tube through a spring and an electrode bush. This provides the electrical link to the control unit.
The inner and outer surfaces of the ceramic tube are coated with porous platinum. The side facing the exhaust gas has a highly porous ceramic layer on top of the platinum, which lets oxygen through.
The ceramic tube with its platinum electrodes is now a porous, solid electrolyte. At temperatures around 350°c or 662°F, it becomes a conductor. One side detects the level of oxygen in the exhaust gas. The other detects its level in ordinary air. If the levels are different, a voltage is generated between the 2 sides.
The control unit compares this voltage to a pre-set level. Below the level indicates a lean mixture, above it means a rich mixture. The control unit may then adjust the pulse to the injectors, to maintain correct mixture. This fine tuning is needed for the catalytic converter to function properly.
Some sensors have a built-in heating element powered by the vehicle’s electrical system. It helps them reach operating temperatures quickly.