Toyota Pick-ups, Land Cruiser, and 4 Runner 1997-00

Oxygen Sensor (HO

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Handle any oxygen sensor with care. Protect the end of the sensor from shock. The sensor can be easily damaged by impact or rough handling. Many technicians feel that 'A dropped sensor is a bad sensor.' Additionally, do NOT allow the sensor to come in contact with water or petroleum products.

OPERATION





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Fig. Common Toyota Oxygen Sensor (HO 2 S)

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), your Toyota truck is equipped with three-way catalytic converters. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. To get the most efficient operation from the converter, the air-fuel ratio must be precisely controlled so it is always near the ideal air-fuel ratio. The ECM has the ability to monitor this process using the oxygen sensor mounted in front (upstream) of the converter and the oxygen sensor mounted after (downstream) of the converter.

The oxygen sensor produces an output voltage that varies with the oxygen content of the exhaust stream. A small electrical signal is generated and input to the ECM. This informs the ECM of the oxygen content (air-fuel ratio) in the exhaust. If the air-fuel ratio is lean, the oxygen content in the exhaust increases and the oxygen sensor output will be low (near zero volts). If the air-fuel ratio is richer than ideal, the oxygen content will be less and the oxygen sensor output will be higher (near one volt). The ECM then calculates the proper duration of fuel injector opening. The oxygen sensor voltage can be monitored with a scan tool

The oxygen sensor is a primary input to the ECM. Like most all sensor inputs and outputs on computer-controlled engines, the voltages are very small. The components, wiring and especially the connectors must be protected from damage.

Your Toyota truck uses Heated Oxygen Sensors (HO 2 S). A heater, controlled by the ECM, warms the Zirconia element. This helps the system operate more efficiently at low temperatures when the engine is first started.

Toyota trucks can use up to four HO 2 S sensors, depending on the engine. Some sensors thread into their mounts, others are retained by two studs and nuts. HO 2 S sensors are typically installed before and after the catalyst. The front (upstream) sensor produces an output signal that indicates the amount of oxygen present in the exhaust gas entering the three-way converter. The rear (downstream) sensor produces an output signal that indicates the oxygen storage capacity of the catalyst; this, in turn, indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the upstream signal will be far more active than that produced by the downstream sensor.

The ECM compares the waveform of the sensor before the catalyst (upstream) with the waveform of the sensor after the catalyst (downstream) to determine whether or not the catalyst performance has deteriorated. Air-fuel ratio feedback compensation keeps the waveform of the oxygen sensor before the catalyst repeatedly changing back and forth from rich to lean. If the catalyst is functioning normally, the waveform of the sensor after the catalyst switches back and forth between rich and lean much more slowly than the waveform before the catalyst. If both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.

A failed HO 2 S sensor should set a Diagnostic Trouble Code (DTC).

TESTING



  1. Start the engine and bring it to normal operating temperature, then run the engine above 1,200 rpm for two minutes.
  2.  
  3. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the HO 2 S and battery ground.
  4.  
  5. Verify that the HO 2 S voltage fluctuates rapidly between 0.400.60 volts.
  6.  
  7. If the HO 2 S voltage is stabilized at the middle of the specified range (approximately 0.450.55 volts) or if the HO 2 S voltage fluctuates very slowly between the specified range (HO 2 S signal crosses 0.5 volts less than 5 times in ten seconds), the HO 2 S may be faulty.
  8.  
  9. If the HO 2 S voltage stabilizes at either end of the specified range; the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a leaking fuel injector. These types of mechanical problems will cause the HO 2 S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired, then the HO 2 S test repeated.
  10.  
  11. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the HO 2 S to detect a lean mixture condition. Reattach the vacuum hose.
  12.  
  13. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the HO 2 S to detect a rich mixture condition.
  14.  
  15. If the HO 2 S voltage is above or below the specified range, the HO 2 S and/or the HO 2 S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
  16.  

REMOVAL & INSTALLATION



  1. Disconnect the negative battery cable.
  2.  
  3. Ensure that the engine and exhaust pipes are cold. Locate the oxygen sensor in question. There should be one in front of the catalytic converter and another behind the catalytic converter. Vehicles with dual front pipes will have four oxygen sensors, depending on the engine.
  4.  
  5. Some oxygen sensors thread into the pipe, others mount with two studs and nuts. Spray penetrating oil on the studs to ease removal.
  6.  



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Fig. This O2 sensor is mounted with two studs and nuts. Use a penetrating oil to ease the removal of the mounting nuts



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Fig. Use a closed end wrench to remove the mounting nuts



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Fig. Unplug the O2 sensor harness



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Fig. It may be necessary to use a deep socket to remove the mounting nuts

  1. Detach the oxygen sensor wiring.
  2.  
  3. Remove the oxygen sensor retaining nuts, if used, or with a suitable wrench, unscrew the sensor if it is the thread-mount type.
  4.  
  5. Remove the oxygen sensor and gasket.
  6.  



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Fig. Remove the sensor from the pipe



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Fig. Remove the gasket from the studs . . .



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Fig. . . then discard the gasket and replace with a new one during installation



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Fig. Most Toyota oxygen sensors are of the 4wire type



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Fig. Typical Toyota oxygen sensor wiring harness

To install:

  1. Install the oxygen sensor with a new gasket. If a thread-type sensor, apply an even coat of anti-seize compound only to the threads. If a stud-mount type sensor, apply a coating of anti-seize compound to the mounting studs. Install the nuts and tighten to 14 ft. lbs. (20 Nm).
  2.  
  3. Connect the oxygen sensor wiring.
  4.  
  5. Connect the negative battery cable.
  6.  

 
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