Oxygen Sensor

The oxygen sensor is mounted on the center exhaust pipe between the turbocharger and the rear exhaust pipe. It is used to sense oxygen concentration in the exhaust gas. If the air/fuel ratio is leaner than stoichiometric (14.7:1), the exhaust gas contains more oxygen. If the air/fuel ratio is richer than stoichiometric, the exhaust gas hardly contains any oxygen.

Therefore, examination of the oxygen concentration in exhaust gas makes it possible to show whether the air/fuel ratio is leaner or richer than stoichiometric. The oxygen sensor has a zirconia tube (ceramic) which generates voltage if there is a difference in oxygen concentration between the inside and outside of the tube.

Platinum is coated on the inside and outside of the zirconia tube for the purpose of catalysis and electrode preservation. The hexagon screw on the outside is grounded to the exhaust pipe, and the inside is connected to the MPFI control unit through the harness.

When rich air/fuel mixture is burnt in the cylinder, the oxygen in the exhaust gas reacts almost completely through the catalytic action of the platinum coating on the surface of the zirconia tube. This results in very large difference in the oxygen concentration between the inside and outside, and the voltage generated is relatively large.

When a lean air/fuel mixture is burnt in the cylinder, oxygen remains in the exhaust gas even after the catalytic action, and this results in a small difference in the oxygen concentration. The voltage generated is relatively small. The difference in oxygen concentration changes greatly near stoichiometric. Thus, the change in the electromotive force is also large.

By sending this information to the MPFI control unit, the air/fuel ratio of the supplied mixture can be determined easily. The oxygen sensor does not generate much voltage when the temperature is low. The characteristics of the voltage stabilize at temperatures of approximately 572°F to 752°F (300°C to 400°C).