Exhaust Oxygen Sensors

Toyota engines utilize two different types of oxygen sensors. The zirconium dioxide sensor is used on all engines except the '90 and later 4A-GE Federal and 3VZ-E California 2WD truck engines. These two engines use a titania oxide sensor.

To bring the system to closed loop operation more rapidly, many engines use a heated exhaust oxygen sensor. The heated sensor provides more accurate exhaust sampling during idle and low speed operation when exhaust temperatures are relatively low. Use of a heated sensor allows closed loop operation earlier during engine warm-up cycles and also allows more flexibility in oxygen sensor location. These factors help in meeting strict exhaust emissions control standards.

Engines produced for sale in California also incorporate a Sub-Oxygen Sensor which helps improve the efficiency of the catalyst system. This sensor is located after the catalyst and is used to fine tune the air/fuel ratio delivered by the injectors, helping to optimize catalyst efficiency.

Zirconium Dioxide Sensor The zirconium dioxide oxygen sensor is an electro-chemical device which compares the oxygen content of the exhaust stream with the oxygen in an ambient air sample. It consists of a zirconium dioxide (Zr02) element sandwiched between two platinum electrodes.

This sensor behaves very similar to a single cell battery. The electrodes act as the positive (+) and negative (-) plates, and the zirconium dioxide element acts as the electrolyte.

Rich air/fuel ratio: If the oxygen concentration on the inside plate differs greatly from that on the outside plate, as it would with a rich air/fuel ratio, electrons will flow through the Zr02 element to the plate exposed to the high oxygen concentration. During rich operating conditions, the inside, or positive plate, is exposed to a much higher concentration of oxygen than the outside, or negative plate. This creates a difference in electrical potential, or voltage, which is measured by a comparator circuit in the E CU.

Lean air/fuel ratio: When the air/fuel ratio becomes lean, the oxygen content of the exhaust gas increases significantly. Because both plates are now exposed to a relatively high concentration of oxygen, electrons balance equally between the two plates. This eliminates the electrical potential between the plates.

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