A Detailed Guide To How Oxygen Sensors Work & What They Do
What is an Oxygen Sensor?
Oxygen sensors (commonly referred to as an “O2 sensor”, as O2 is the chemical formula for oxygen) are 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. The cost to replace an oxygen sensor ranges from $155-$500, depending on if you DIY or go to a shop.
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
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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.