Toyota Celica/Supra 1971-1985 Repair Guide

Oxygen Sensor System



On some engines, the oxygen sensor is used to control the air injection system. It is not used for feedback fuel control system.

The three-way catalytic converter, which is capable of reducing HC, CO and NOx into CO 2 , H 2 O, O 2 and N 2 can only function as long as the air/fuel mixture is kept within a critically precise range. The oxygen sensor system is what keeps the oxygen range in control.

Basically, the oxygen sensor system works like this: As soon as the engine warms up, the computer begins to work. The oxygen sensor, located in the exhaust manifold, senses the oxygen content of the exhaust gases. The amount of oxygen in the exhaust varies according to the air/fuel mixture. The O 2 sensor produces a small voltage that varies depending on the amount of oxygen in the exhaust at the time. This voltage is picked up by the computer. The computer works together with the fuel distributor and together they will vary the amount of fuel which is delivered to the engine at any given time.

If the amount of oxygen in the exhaust system is low, which indicates a rich mixture, the sensor voltage will be high. The higher the voltage signal sent to the computer, the more it will reduce the amount of fuel supplied to the engine. The amount of fuel is reduced until the amount of oxygen in the exhaust system increases, indicating a lean mixture. When the mixture is lean, the sensor will send a low voltage signal to the computer. The computer will then increase the quantity of fuel until the sensor voltage increases again and then the cycle will start all over.


See Figures 1, 2 and 3

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Fig. Fig. 1: The sensor is secured to the exhaust manifold with two bolts

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Fig. Fig. 2: Once the bolts are removed, pull the sensor out ...

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Fig. Fig. 3: ... then disconnect the harness

  1. Disconnect the negative battery cable.
  3. Unplug the wiring connector leading from the O 2 sensor.

Be careful not to bend the waterproof hose as the oxygen sensor will not function properly if the air passage is blocked.

  1. Unscrew the two nuts and carefully pull out the sensor.
  3. Installation is in the reverse order of removal. When installing the sensor, always use a new gasket. Tighten the nuts to 13-16 ft. lbs. (17-22 Nm).