Toyota Pick-ups/Land Cruiser/4Runner 1970-1988

Oxygen Sensor 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 Figure 1

  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. Please note the following:

    Always use a new gasket.
    Tighten the nuts to 13-16 ft. lbs.


Click image to see an enlarged view

Fig. Fig. 1: Removing the oxygen sensor


See Figures 2 and 3

All models are equipped with an oxygen sensor warning light on the instrument panel. The light may go on when the truck is started, then it should go out. If the light stays on, check your odometer. The light is hooked up to an elapsed mileage counter which goes off every 30,000 miles. This is your signal that it is time to replace the oxygen sensor and have the entire system checked out. After replacement of the sensor, the elapsed mileage counter must be reset. To reset:

  1. Locate the counter. It can be found under the left side of the instrument panel, on the brake pedal bracket. On some Land Cruiser models, the counter is located under the hood near the firewall area.
  3. Unscrew the mounting bolt, disconnect the wiring connector and remove the counter.
  5. Remove the bolt on top of the counter.
  7. Lift off the counter cover and push the reset switch.

The warning light on the instrument panel must go out at this time.

  1. Installation is in the reverse order of removal.

Click image to see an enlarged view

Fig. Fig. 2: Remove the counter cover

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Fig. Fig. 3: Push the switch to reset