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OPERATION
Fig. Once combustion occurs, the exhaust gases travel through the exhaust manifold and pass across the tip of the oxygen sensor.
Fig. Dissection of a typical oxygen sensor
Fig. Typical oxygen sensor element
Fig. Most heated oxygen sensors used on OBD II type engine management systems, such as the one on your Camry or Avalon, have four wire connectors
Fig. Location of the oxygen sensor-5S-FE engine
Fig. Use a ratchet and a deep socket to remove the oxygen sensor mounting nuts-5S-FE engine
Fig. Remove the oxygen sensor from the exhaust manifold-5S-FE engine
Fig. Always replace the oxygen sensor gasket-5S-FE engine
Fig. Remove the heat shield . . .
Fig. . . . to gain easier access to the oxygen sensor and the intake manifold.
The exhaust oxygen sensor or O2S, is mounted in the exhaust stream where it monitors oxygen content in the exhaust gas. The oxygen content in the exhaust is a measure of the air/fuel mixture going into the engine. The oxygen in the exhaust reacts with the oxygen sensor to produce a voltage which is read by the ECM.
There are two types of oxygen sensors used in these vehicles. They are the single wire oxygen sensor (02S) and the heated oxygen sensor (H02S). The oxygen sensor is a spark plug shaped device that is screwed into the exhaust manifold. It monitors the oxygen content of the exhaust gases and sends a voltage signal to the Electronic Control Module (ECM). The ECM monitors this voltage and, depending on the value of the received signal, issues a command to the mixture control solenoid on the carburetor to adjust for rich or lean conditions.
The heated oxygen sensor has a heating element incorporated into the sensor to aid in the warm up to the proper operating temperature and to maintain that temperature.
The proper operation of the oxygen sensor depends upon four basic conditions:
TESTING
Heated Oxygen Sensor
- Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
- Turn the ignition OFF disengage the H02S harness connector.
- Connect an ohmmeter between terminals +B and HT of the sensor. Resistance should be 11-16 ohms at 68° F (20° C). If not, replace the sensor.
Fig. Check the resistance of the heater portion of the sensor
- Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
- Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) signal wire and battery ground.
- Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
- If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
- If the 02S 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 faulty fuel pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
- 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 02S to detect a lean mixture condition. Reattach the vacuum hose.
- 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 02S to detect a rich mixture condition.
- If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
Fig. Heated oxygen sensor (Bank 1, Sensor 1)-5S-FE engine except California
Fig. Heated oxygen sensor (Bank 1, Sensor 2)-5S-FE engine except California
The oxygen sensor can be located in several places. Either in the exhaust manifold, front pipe or catalytic converter.
- The best condition in which to remove the sensor is when the engine is moderately warm. This is generally achieved after two to five minutes (depending on outside temperature) of running after a cold start. The exhaust manifold has developed enough heat to expand and make the removal easier but is not so hot that it has become untouchable. Wearing heat resistant gloves is highly recommended during this repair.
- With the ignition OFF , unplug the connector for the sensor.
- Remove the two sensor attaching bolts. Some sensors do not use the two bolts and are threaded instead (looks similar to a spark plug). If this is the case, use a wrench to unscrew the sensor. A special socket with one side cut out (to allow the wire to hang out) is also available.
- Remove the oxygen sensor from its mounting place and discard the gasket.
Fig. A special socket is available to remove the threaded type sensor
To install:
- Apply a coat of anti-seize compound to the bolt threads but DO NOT allow any to get on the tip of the sensor.
- Position a new gasket, install and secure the sensor.
- Reattach the electrical wiring and insure a clean, tight connection.
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