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 4

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.

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.