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Click the image for a more detailed explanation.
Click the image for a more detailed explanation.
Click the image for a more detailed explanation.
Click the image for a more detailed explanation.
Click the image for a more detailed explanation.
Click the image for a more detailed explanation.

Step 1

If your vehicle has illuminated a check engine light, CEL or service engine soon SES light, you can find the problem code with a hand held device. Many auto parts stores will read the code for free if you ask.

Step 2

The hand held device will give a brief description of the symptoms, but an internet search will give a more complete picture of the possible causes.

Step 3

If you have determined that you need to replace the oxygen sensor, immobilize a wheel on your vehicle before raising it.

Step 4

Almost all connectors for oxygen sensors have a plastic locking tab. A small flat blade screwdriver is a handy tool for depressing the tab so the connector can be unplugged.

Step 5

The oxygen sensor is removed with a counter clockwise rotation. Sometimes, the sensor is difficult to remove because of corrosion.

Step 6

Anti-seize is provided and should be applied only on the threads of the sensor. The anti-seize eases the removal of the sensor during a future repair.

Step 7

The sensor is installed with about 35 ft-lbs of torque. Excess torque will distort the shell of the sensor and cause immediate sensor failure. Apply just enough torque to squeeze the crush washer on the sensor.

Step 8

When the sensor connector is fully plugged into the vehicle harness, you might hear or feel a click. This is the locking tab, holding the connectors together.

Step 9

After the sensor installation, the problem code is still stored in the computer. Use the handheld tool to clear the code. If the oxygen sensor was the correct solution to the problem, the code will not re-appear.

Step 10

Lower the vehicle, and remove the wheel blocks. Happy and safe driving to you!
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General FAQ

What does an oxygen sensor do?

The oxygen sensor monitors the engines air to fuel ratio by measuring the amount of free oxygen in the exhaust. It reports this information to the engine control unit or ECU. Approximately every 10 milliseconds the ECU uses this information to make corrections to the air to fuel mixture for maximum efficiency.

What is an oxygen sensor made of?

An oxygen sensor is a solid electrolyte galvanic cell. Electrolyte is zirconium oxide, which is the same material synthetic diamonds are made from. This material is stabilized with a rare element called Yttria. The cell (thimble or flat plate) is then coated with pure platinum, which acts as a conductor/electrode (like battery plates).

How does the oxygen sensor work?

The sensor works as a result of the varying quantities of oxygen in the exhaust verses the amount in the atmosphere. Voltage is produced by the difference in the two amounts. If the amount of oxygen in the exhaust is closer to the amount in the air, the engine is lean and the voltage is low (under 250mv). If the engine is rich the voltage is high (about 950mv).

Are all oxygen base sensors generally the same?

No. The base sensor design and elements vary greatly. Below is a list of the most widely used base sensors.

  • Single wire unheated standard zirconia sensors.
  • Two wire unheated standard zirconia sensors.
  • Three wire unheated standard zirconia sensors.
  • Three wire heated titania sensors.
  • Four wire heated standard zirconia thimble type isolated ground sensors. Within this sensor group there are multiple heater types.
  • Four wire heated standard zirconia thimble type case ground sensors. Within this sensor group there are multiple heater types.
  • Four wire heated standard zirconia planar type isolated ground sensors. Within this sensor group there are multiple heater types.
  • Four wire heated zirconia thimble type air fuel ratio sensors.
  • Four wire heated zirconia planar type air fuel ratio sensors.
  • UEGO five wire wide air fuel ratio sensors.

What is a universal oxygen sensor?

A universal sensor is a base sensor, which does not include the direct fit connector. Splicing is necessary for installation. Universals were widely used in the early years of vehicles equipped with oxygen sensors.

The possibility exists that the user could select the wrong base sensor by assuming that all sensors with the same wire count are equal. All Oxygen Sensors are not created equally. Each type is matched to the make, model and sub-model application and cannot be mixed. It is impractical to offer a universal sensor for many applications due to heater types, ground types and other characteristics. Improper selection of the universal sensor could result in serious damage to the engine management system, including failure of the engine control unit (ECU) and/or the catalytic converter.

What will damage an oxygen sensor?

Natural aging, shock from accidents, antifreeze poisoning, excessive oil consumption or leakage, silicone poisoning due to incorrect use of silicone gasket sealers, etc.

What do oxygen sensor wire colors mean?

Typically the colors depict the connections inside the sensor; however, various manufacturers choose different colors. For example; some manufacturers use white wires for the heater & others use black. For this and many other reasons it is best to use a direct fit connector to avoid confusion.

Technical FAQ

My oxygen sensor will not come out.

To more easily remove an oxygen sensor, soak the sensor thread area with a powerful penetrating lubricant. Starting and revving the engine should further aid in loosening the sensor by heating up the bung. If you are using an open end wrench, try an O2 socket.

If this fails, try a long ratchet or breaker bar in conjunction with your socket to generate more torque. If you are still unsuccessful, heat the bung with a torch until cherry red and remove the sensor. After the sensor is removed, be sure to use a thread cleaner to clean up the bung threads.

In some cases the threads will need to be repaired. This can be done with a thread repair kit (Walker Part # 88-832). Do not use an impact wrench to remove an O2 sensor, as you will most likely strip the threads in the bung.

How many oxygen sensors does my vehicle have?

Our Find Your Part lookup on the right can give you specific sensor data for your vehicle. Modern cars can have up to 6 or more O2 sensors.

Where exactly are my oxygen sensors located?

In some cases, your sensors will be easily visible on your exhaust. A more detailed description and diagrams of common O2 sensor locations can be found on this page.

The best way to determine the exact locations for your vehicles sensors is to consult a maintenance manual such as those put out by Haynes or Chilton.

Do I really need the rear oxygen sensors?

The role of your downstream sensors is to monitor the output and health of your catalytic converter. Removing them will take away this function, and produce a CEL (check engine light) or MIL (malfunction indicator light) on your vehicle.

I am getting a CEL/MIL and a && code. Do I need to replace my oxygen sensors?

Not necessarily. The oxygen sensor simply reports the data that it gathers. For example, if you are getting a lean mixture code, you may have a vacuum leak or a faulty fuel injector. Replacing the oxygen sensor will not fix this problem. You will just get the same code again. You can find more information about diagnostic codes for your oxygen sensors on this page.

Do I need to replace all of my oxygen sensors at once?

It is best to replace your sensors in pairs. For example, if you replace the downstream left sensor, you should also replace the downstream right. However, on most vehicles produced since 1996, replacing one sensor (especially the front engine monitoring sensor) will cause the ECU to set a code for the other sensors.

This is because the new sensor switching activity is much faster than that of the older aged sensors. It is probable that on most vehicles, the code will be set within 30-60 days AFTER the first sensor replacement.

I pulled my oxygen sensor and it is an orange/black/white color. What does this mean?

An orange hue indicates lead poisoning, black indicates carbon buildup, and white can be a sign of silicone poisoning or antifreeze contamination. Repairs should be made at the source of the trouble, and the sensors need to be replaced. A more complete list of oxygen sensor symptoms and their causes can be found here.

What is the life expectancy of an oxygen sensor?

Heated oxygen sensors should be checked or replaced every 60,000 miles, while unheated or one wire oxygen sensors should be checked or replaced every 30,000 miles. See our oxygen sensor page.

How can I test my oxygen sensor?

You can test the O2 sensor on a vehicle by first identifying the signal wire on the sensor. Secondly, by using a voltmeter with the scale set to 1 volt, the voltage will fluctuate between 200 and 800 millivolts or .2 to .8 volts on your meter. If the sensors reading is stalled in position, or switches abnormally high or low, your sensor has failed. If your results are inconclusive, its best to have your vehicle checked at a professional shop.
Note: This test will not work on Air Fuel or Wide Band sensors.

A second method is to connect some of the various testers available on the market directly to the oxygen sensor. This method is not as accurate, but can detect some of the sensor failures.

What is a California emissions sensor? How do I know if I need one?

A California emissions O2 sensor is meant for vehicles that are designed to meet California emission regulations. Such vehicles should have a sticker under the hood or on the drivers door jamb that identify them.

What are the symptoms of a failing oxygen sensor?

Typically, a failing sensor will produce poor gas mileage, hesitation or stalling, and a CEL/MIL. However, these symptoms could also be caused by something other than the oxygen sensor.

How do I locate specified sensor locations (B1S1, downstream left, etc.)?

Bank 1, containing cylinder # 1, is always the most forward cylinder on the block. Finding Bank 1 is not difficult. The front of the engine will have the accessory pulleys and drive belts, regardless of orientation in the engine compartment. There will be a visible difference in the cylinder head location.

Refer to the diagrams on this page. Sensor 1 will be the pre-catalytic position and Sensor 2 would typically be the post-catalytic position. In some instances, Sensor 2 can be pre-catalytic and thus making Sensor 3 post-catalytic.

Left and right sensor positions are found in reference to the rear of the engine (the side opposite of the belts). Upstream (pre-cat) and downstream (post-cat) are found in reference to the catalytic converter.

See more detailed diagrams and description on our on this page.

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Oxygen Sensor Locations

Oxygen sensors can be found in a variety of places.
Below are some of the more common locations.

Traditional V6 & V8

4 Cylinder Transverse

V6 & V8 Transverse

Left or right bank is determined viewing from the rear of the engine (opposite the belts).

4 & 6 Cylinder In-line

Sensor location for split manifold is shown.
For a single manifold, there is only one pre-cat sensor.

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Common O2 Sensor Conditions

For the end user and vehicle repair shops performing a tune-up,
remove the oxygen sensor and check for these symptoms:

New Oxygen Sensor

Lead Poisoning

Ethylene Glycol~(Antifreeze) Contamination

Rich Fuel Mixture

Silicone Poisoning

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Oxygen Sensor(O2) Information


The oxygen sensor is a device which determines the oxygen content of the exhaust gas. Since the amount of oxygen in the exhaust gas is a very good indicator of combustion efficiency, it is also the best place to monitor the air fuel ratio.

Located in the exhaust system, the oxygen sensor produces a voltage proportional to the amount of oxygen in the exhaust versus the air (<150 mV lean & >750 mV rich). This data is used to control the air-fuel mixture through PORT or TFI injectors and carburetors. Sensors are monitored or checked anywhere from 4 to 100 times per second. The air-fuel mixture is thus always moving from rich to lean averaging very close to stoichiometric (ideal) ratios.

The ceramic sensor body is contained in a housing which protects it against mechanical effects and facilitates mounting. The ceramic body is made of stabilized zirconium dioxide (zirconia). Its surfaces are coated with electrodes made of a gas-permeable platinum layer. In addition, a porous ceramic coating has been applied to the side exposed to the exhaust gas. This coating prevents contamination and erosion of the electrode surfaces by combustion residue and particulates in the exhaust gases.

Walker Oxygen Sensors...

..are precision made for outstanding performance and manufactured to meet and exceed all original equipment specification and test requirements.
  • Improved Engine Response & Performance
  • Lower Emissions
  • Improved Fuel Economy
  • Three wire heated titania sensors.
  • Longer Sensor Life
  • Meets OEM Specifications

Malfunctioning / Failed Sensors

Oxygen sensors can fail when the sensors ceramic element is exposed to certain types of silicone compounds or when an oil-burning engine adds to the sensor becoming oil-fouled. Small amounts of tetra-ethyl lead in the gasoline or over-the-counter fuel additives, which are not oxygen sensor safe, can also kill an oxygen sensor.

Failures can occur instantaneously at the time the contaminant contacts the oxygen sensor, causing a dead sensor, or gradually over a period of time. Gradual deterioration results in a slow sensor which does not react as quickly as it should, causing the catalytic converter to perform less efficiently. This can lead to premature failure of the catalytic converter.

Slow oxygen sensors can cause a drop in fuel economy of 10-15%, excessive exhaust emissions and poor drivability. Unfortunately, the symptoms of a slow oxygen sensor are not always obvious to the vehicle owner unless the vehicle fails an emissions test, a decline in fuel economy is noticed or drivability problems occur.

A dead sensor can be detected with a relatively inexpensive digital volt-ohm meter. A slow sensor can only be diagnosed by using a digital oscilloscope or scope meter. Most installers will probably not be able to spot an oxygen sensor problem until it is too late and the catalytic converter is already well on its way to failure.

Preventive Maintenance

One-wire and two-wire unheated oxygen sensors should be checked or replaced every 30,000 to 50,000 miles. These sensors rely solely on hot exhaust gas to heat up to operating temperature and are designed to allow a large volume of exhaust gas to make contact with the active ceramic element. These sensors are exposed to contamination, especially the wide-slot varieties found on Chrysler, Ford and General Motors vehicles.

Heated oxygen sensors have a built-in heater which heats the sensors. Much less exhaust gas needs to contact the ceramic element, making these sensors less prone to contamination.

Heated sensors can also be located further downstream (closer to the catalytic converter) which increases their life expectancy. Heated oxygen sensors should be checked or replaced every 60,000 to 100,000 miles.

Diagnostic Trouble Codes (DTC)

When oxygen sensor failure occurs, a DTC is recorded in the Engine Control Unit (ECU) and a Malfunction Indicator Lamp (MIL) is illuminated on the dash, alerting the driver the vehicle has a problem.

To diagnose the fault, a code reader or scan tool is connected to the vehicle to read the trouble code. The scan tools can vary in the display of information shown. Some show a definition for the code while others show only a trouble code number. There are generic or standard OBD II codes and vehicle manufacturers use additional codes called enhanced or OEM-specific codes. On many older (pre 1995) vehicles, a trouble code or DTC can be read without a scan tool or code reader by using a manual flash code procedure.

After having identified the description using a list of DTC trouble codes, the next step is to diagnose the fault. You must follow the diagnostic procedure to properly diagnose the system, sensor and/or circuit.

The trouble code itself does not tell you which part to replace! The scan tools or code readers speak OBD II language, meaning the references to the engine are coded.

  • B1 = Bank 1
  • B2 = Bank 2
When applying OBD II to oxygen sensors, the codes would then read B1S1.
This refers to Bank 1 Sensor 1.

Finding Bank 1 is not difficult. The front of the engine will have the accessory pulleys and drive belts, regardless of orientation in the engine compartment. Bank 1, containing cylinder #7, is always the most forward cylinder on the block. There will be a visible difference in the cylinder head location.

Sensor 1 will be the pre catalytic position and Sensor 2 would typically be the post catalytic position. In some instances, Sensor 2 can be pre catalytic thus making Sensor 3 post catalytic.

Life Expectancy

30,000 to 50,000 miles is typical. However, constantly exposed to the harsh environment found in an automobiles exhaust system, the oxygen sensor sustains a constant barrage of harmful exhaust gases, extreme heat and high velocity particulates. And that is under normal operating conditions!

Sometimes contaminants such as coolant, oil or silicone particulates will find their way to the sensor as well. These contaminate the sensor and render it inoperable. An oxygen sensors life is long, in some applications up to 100,000 miles. Whether by contamination or normal use, its effectiveness will inevitably decrease over time.

At Walker Products, we recognize the need to keep vehicles running clean. A bad oxygen sensor can cause unacceptable emissions levels, affect performance and ultimately damage the catalytic converter. Make it a point to check the oxygen sensors at each tune-up and replace faulty sensors with new Walker oxygen sensors.

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