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Oxygen Sensors: A Detailed Guide To How Oxygen Sensors Work & What They Do

Reading Time: 5 minutes

What is an Oxygen Sensor?

The oxygen sensor (commonly referred to as an “O2 sensor”, as O2 is the chemical formula for oxygen) is mounted in the exhaust manifold of the vehicle to monitor how much unburned oxygen is in the exhaust as the exhaust exits the engine.

By monitoring oxygen levels and sending this information to your engine’s computer, these sensors let your car know if the fuel mix is running rich (not enough oxygen) or lean (too much oxygen). Proper air fuel ratio is critical to keeping your car running as smoothly as it should be.

Since the O2 sensor plays an important role in engine performance, emissions, and fuel efficiency, it’s important to understand how they work and ensure yours are working properly.

Where are Oxygen Sensors Located?

The amount of oxygen sensors a vehicle has varies. Every car made after 1996 is required to have an oxygen sensor upstream and downstream of each catalytic converter. Therefore, while most vehicles have two oxygen sensors, those V6 and V8 engines equipped with dual exhaust have four oxygen sensors — one upstream and downstream of the catalytic converter on each bank of the engine.

What does an Oxygen Sensor do?

The vehicle’s 02 sensor is used to measure the amount of oxygen in the exhaust and report this feedback to your vehicle’s computer. The computer then uses this information to adjust your air/fuel mixture.

Oxygen sensors work by producing their own voltage when they get hot (approximately 600°F). On the tip of the oxygen sensor that plugs into the exhaust manifold is a zirconium ceramic bulb. The inside and the outside of the bulb is coated with a porous layer of platinum, which serve as the electrodes. The interior of the bulb is vented internally through the sensor body to the outside atmosphere.

When the outside of the bulb is exposed to the hot gases of the exhaust, the difference in oxygen levels between the bulb and the outside atmosphere within the sensor causes voltage to flow through the bulb.

If the fuel ratio is lean (not enough fuel in the mixture), the voltage is relatively low — approximately 0.1 volts. If the fuel ratio is rich (too much fuel in the mixture), the voltage is relatively high — approximately 0.9 volts. When the air/fuel mixture is at the stoichiometric ratio (14.7 parts air to 1 part fuel), the oxygen sensor produces 0.45 volts.

The Upstream Oxygen Sensor (Oxygen Sensor 1)

Oxygen sensor 1 is the upstream oxygen sensor in relation to the catalytic converter. It measures the air-fuel ratio of the exhaust coming out of the exhaust manifold and sends the high and low voltage signals to the powertrain control module in order to regulate the air-fuel mixture. When the powertrain control module receives a low voltage (lean) signal, it compensates by increasing the amount of fuel in the mixture. When the powertrain control module receives a high voltage (rich) signal, it leans the mixture by reducing the amount of fuel it adds to the mixture.

The powertrain control module’s use of the input from the oxygen sensor to regulate the fuel mixture is known as a closed feedback control loop. This closed loop operation results in a constant flip-flop between rich and lean, which allows the catalytic converter to minimize emissions by keeping the overall average ratio of the fuel mixture in proper balance.

However, when a cold engine is started, or if an oxygen sensor fails, the powertrain control module enters into open loop operation. In open loop operation, the powertrain control module does not receive a signal from the oxygen sensor and orders a fixed rich fuel mixture. Open loop operation results in increased fuel consumptions and emissions. Many newer oxygen sensors contain heating elements to help them get to operating temperature quickly in order to minimize the amount of time spent in open loop operation.

The Downstream Oxygen Sensor (Oxygen Sensor 2)

Oxygen sensor 2 is the downstream oxygen sensor in relationship to the catalytic converter. It measures the air-fuel ratio coming out of the catalytic converter to ensure the catalytic converter is functioning properly. The catalytic converter works to maintain the stoichiometric air-fuel ratio 14.7:1 while the powertrain control module constantly flip-flops between rich and lean air-fuel mixtures due to the input from the upstream oxygen sensor (sensor 1). Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts.

Symptoms of a Bad O2 Sensor

When an 02 sensor fails, there are a variety of diagnostic trouble codes (DTCs) that can pop up. Most of the time, a faulty O2 sensor will result in a check engine light accompanied with a fault code that you can read with an OBD2 scanner like FIXD. Based on this fault code, it will point to how it failed and then move forward with the diagnosis. 

Symptoms of a faulty O2 sensor may include the following:

  • Lean or rich running condition 
  • Poor acceleration
  • Engine hesitation
  • Black smoke from tailpipe (rich running condition) black smoke is excess fuel exiting the exhaust
  • Rough idle
  • Vehicle stalling out
  • Reduced fuel efficiency

To determine if you have a bad oxygen sensor vs. a lean or rich running condition, the first step is to check the operation of your O2 sensor with a scan tool

How to Test Oxygen Sensors

Since the O2 sensor plays an important role in keeping your engine running as efficiently and cleanly as possible, it’s important to make sure that it’s working properly. Most oxygen sensors typically last 30,000 to 50,000 miles, or 3-5 years, with newer sensors lasting even longer with proper maintenance and upkeep.

You can test the oxygen sensor at home with a voltmeter or OBD2 scan tool like the FIXD Sensor. Go to the live data feed within the FIXD app to see the voltage and response time of your O2 sensors. 

Typically, a front (upstream) O2 sensor 1 that is functioning properly will be switching from rich to lean at a fairly steady rate, creating a wavelike formation. The voltage generated from the O2 sensor should be from 0.1V to 0.9V, with 0.9V on the rich side and 0.1V on the lean side. If your readings are within this range, the O2 sensor is functioning properly.

The rear (downstream) oxygen sensor 2 is a catalyst monitor and if everything is operating normally, this sensor will be hovering around half a volt. However, this measurement can fluctuate depending on the manufacturer.

Additional O2 Sensor Testing Tips

If the O2 Sensor is not responding quickly to testing:

If the sensor seems sluggish or slow to respond during testing and there are other symptoms without a fault code, this may be an issue of a “lazy” O2 sensor that can cause other problems.

If the O2 Sensor voltage is sticking rich or lean:

Try introducing the opposite condition to determine if the issue is with the oxygen sensor or if it’s an air-fuel mixture issue. For example, if your O2 sensor is sticking lean, add fuel to the situation to see if it responds. If the O2 sensor is on the rich side, try introducing a vacuum leak or more oxygen to see how and if the sensor responds.

Stay in the Know with the FIXD Sensor & App

With the FIXD car scanner and app, you can take control of car care and save $1000s. From automatic maintenance alerts sent straight to your phone, to real-time data showing fuel trims, oxygen sensor levels, battery voltage, & more, FIXD keeps you informed so you can extend the life of your vehicle and avoid unnecessary upsells. Learn more about the FIXD OBD2 scanner and app today!

Wife, mom, Content Manager & Senior Copywriter at FIXD. From the garage to the gym, I love helping people learn and grow. Dream car: ‘69 Acapulco Blue Mustang.

Kate McKnight
Wife, mom, Content Manager & Senior Copywriter at FIXD. From the garage to the gym, I love helping people learn and grow. Dream car: ‘69 Acapulco Blue Mustang.

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    37 Comments

    1. When cooled down check engine went out .all started all full tank of e85

      1. The information was very helpful.thank you very much.

        1. I have a Fixd but can’t get it to connect to my phone

          1. Hi Bobby, so sorry to hear that you are having connectivity issues. Please make sure you have the most updated version of the FIXD App additionally, please make sure you are connecting through the FIXD App and not through the Bluetooth menu of your smartphone. If you have taken these steps and are still experiencing issues, please reach out to our support team here: https://fixd.zendesk.com/hc/en-us/requests/new

    2. hi there,
      thank you very much. the information was realy useful.
      all the best.
      bahrami

      1. How many a/f & oxygen sensors or a 2000 Lexus es300 v6 3.0

        1. how does the o2 sensor reduce consumption of fuelin a moving car?

        2. My mechanic still trying to sell my me a CAT

    3. What if I don’t o2 sencers or cats

    4. Very good article about O2 sensors! Thank you for the schooling, I appreciate it.

      1. My car keeps throwing a code p0420 and the CEL keeps illuminating. I installed a 1 ohm resistor and a 4.7 farad capacitor to fool the computer and now my scanner is reading a code p2097 and CEL is on again. Should I use a 1 farad capacitor instead of the 4.7 farad capacitor? Thanks for the tutorial on O2 sensors, I understand how they work aa little better now.

        1. Your car seems like its on drugs you gave it one thing and here comes the side effects lol

        2. try replacing the f-converter that is what the code is telling you.

    5. Great info. Nice work. Thank you.

      1. I think everytime we get a check engine light it should have a walk through like the o2 sensor part of my problem was a stuck open pcv valve found it by mistake and the hose to it has a sm crack that i could not hear it sucking in air so all my misfires went away but still showing its taking to much air i ordered the airhose

    6. I’m not a mechanic but I do try to do what I can on my own through reading and you tube so this information was very helpful since I cannot seem to get my emissions test passed to get a new inspection sticker and I’m always skeptical of what mechanics tell me so I like to go in there and at the very least sound like I know what I’m talking about and maybe scare them into not trying to rip me off lol

      1. GM quoted me $3300 to change the catalytic converters on my 2011 Chevy Traverse. This price includes all 4 O2 sensors as well! They’re all ripoff artists! Loved your comment…Lmao!!!

        1. I have a 98 ford contour. I only get about 160 miles a tank. I had a code for B1S1 but found a broken wire on another. The light stayed off but I still get crappy mileage. Could it be still B1S1 or could it be upstream sensors?

    7. How do I know which sensor it is-can FIXD tell me so I can just tell mechanic to fix that one and they done rip me off?

      1. Hey Crystal! Depending on which code the FIXD Sensor detects, it will tell you which specific sensor is having trouble.

        Thanks,
        FIXD Team

      2. Hey does your fixd say upstream or downstream if its up its behind the motor if down its buy the cats but you may have more then too . you can take it to advance auto or any O’Reilly’s or auto part that sells batteries and everything else for your vehicle and they can test it for you they can tell you which one it is even give you the part so you can put it on yourself it’s very easy

    8. Hi, my oxygen sensor lambda (bank 1 sensor 1) value starts at 0.9999 when ignition is on, and i can see values going as high as 30? The mean value is 4.9??

    9. How do you clean the mass air flow sensor?

      1. You buy a can of MAF Sensor cleaner at Walmart or Auto store. The part is located inside the intake tube, where the fresh air first comes into your engine. That long plastic hose usually that connects to the throttle body. They sell the MAF/Throttle body cleaner as a two-pack. Make sure you only use the MAF cleaner on the MAF sensor. It’s a simple procedure. A delicate part though, so remove it carefully, On my 09 Mercury Marquis, it was two screws to loosen, then slide it out. Spray her down till it’s dripping. Repeat once for good measure or more if it’s really dirty, but usually the two times does the trick. Hope that helps. As another poster said-YouTube is wealth of informative videos for almost every car made. Good luck!

    10. Your article is only partly correct. The catalytic converter does NOTHING to CONTROL the mixture. All it does is to complete combustion that may not have been completed in the engine.

      Also note that GM and Chrysler used the O2 sensor as a feedback/mixture control device. The primary mixture control component on Ford vehicles was the Mass Air Flow sensor or the B/MAP sensor on older models. Oxygen sensor on Fords was only used as a redundant backup device to verify that the mixture control components were operating correctly.

    11. I have a 1997 ford Taurus GL wagon that I swear has a Gremlin it. I have replaced the IAC valve, and the Mass Air Flow sensor, and both O2 sensors and still the same problem. sitting at a light it will do what seams to be a misfire and i have to put it into neutral. the plugs are new as well as the wires. ant ideas..

      1. Check all air lines throughly check PCV valve as well that stopped alot of my check engine lights

    12. as i’m running gas on my crewman the engine warning light will go off saying it is either running too lean while crusing at 100km or it will go off saying mixture is too rich at lower speeds. however this is intermittant. i’m using a fixd module in the car which assists diagnosing the problem and clearing the light. howwever the problem persists.

    13. Thank you for the information, I do have a question:
      Why are the temperatures in bank 1=600°C & bank 2=500°C are not the same?

    14. Excellent article. Well written and concise.

    15. CHECK YOUR STICKY PVC VALVE. ROUGH IDLE ON START.

    16. I got a 2005 Jeep Grand Cherokee Laredo 3.7L V6. Dual exhaust. How many O2 sensors would I have? And what would I need to replace all of them? Upstream/downstream the specifics.

    17. This information is very helpful to me.

    18. I love the information that I have received,it is very helpful.

    19. Hey I have a 2017 chevy malibu 1.5 turbo and I am trying to find the other oxygen sensor. Does anyone know where it is? I have p0420 and p2097 as codes and I want to clean the my oxygen sensor to see if that works. Thanks in advance for your help.

    20. on a 2010 chevy malibu i would like to gut my catalytic converter and was wondering if it would throw off the second sensor which I think is called downstream O2 sensor. I have a sensor before the catalytic converter which is O2 and then I have a sensor underneath the catalytic converter was wondering if I can keep it plugged in but gut the cat

    21. What would it mean then after replacing both o2 sensors and the cat, that the downstream is showing 1 volt at idle, but fluctuates up and down (like the upstream) during acceleration?

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