Technical Oxygen Sensor FAQ

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Frequently Asked Questions about Oxygen Sensors

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 vehicle’s 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 sensor’s reading is stalled in position, or switches abnormally high or low, your sensor has failed. If your results are inconclusive, it’s 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 driver’s 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.
More information can be found on the oxygen sensor page of our website.

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 below. 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.

V6 & V8 Inline* 4 & 6 Cylinder Transverse*
V6 & V8 Transverse* 4 & 6 Cylinder Inline*
Left or right bank is determined by viewing from the rear of the engine (opposite the belts). Sensor location for a split manifold is shown. For a single manifold, there is only one pre-cat sensor.

*The illustrations above represent typical engine mounting configurations. There are many variations depending on make and model. The buyer should take care to choose the correct sensor for a specific application location.
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