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

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.

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?

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 relationship 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.

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Comments 12

Dwight Daniel

June 6, 2018

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


September 17, 2018

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


September 27, 2019

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

Mike kusch

October 24, 2018

What if I don’t o2 sencers or cats


December 2, 2018

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


February 5, 2019

Great info. Nice work. Thank you.

Danny Moynihan

March 20, 2019

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


March 27, 2019

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?

FIXD Marketing

March 27, 2019

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



May 28, 2019

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??


June 9, 2019

How do you clean the mass air flow sensor?

Jack Mick

July 2, 2019

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!

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