Oxygen Sensors

What is an oxygen sensor?

Oxygen sensors, also commonly referred to as a lambda sensor, an air fuel ratio sensor (AFR or AF), or an O2 sensor, were first used in an automobile in the late 1970s, and are a major component to all modern vehicles. There are many different kinds of oxygen sensors, and each is designed and will function differently. However, all oxygen sensors share the same purpose. Without getting too complex, let’s explore the basics of automotive oxygen sensors to give you a better understanding.

Oxygen sensors monitor the amount of oxygen within the exhaust of a vehicle compared to the amount of oxygen in the air to determine how rich or lean the engine is running, which is known as the air / fuel ratio. This information is converted to voltage and sent to the Engine Control Unit (ECU). When in closed loop mode, the ECU uses this information to make the necessary adjustments in fuel mixture to achieve the ideal air / fuel ratio for the situation. The end goal of an O2 sensor is to help the car or truck’s engine perform as efficiently as possible and to ensure that your vehicle produces as few exhaust emissions as possible.

Where are the O2 sensors located in my vehicle?

Since an oxygen sensor is a device that monitors oxygen levels within the exhaust of a vehicle, they need to be mounted somewhere along the exhaust system. The amount of O2 sensors in your car or truck and their exact location is specific to the year, make, and model. Vehicles built prior to OBD-II (on board diagnostics) specifications will have a sensor (or more) between the engine and the catalytic converter—these are referred to as upstream or front oxygen sensors. Upstream oxygen sensors are commonly located on the exhaust manifold or front exhaust pipe. Vehicles equipped with OBD-II will have a sensor (or more) after the catalytic converter—these are known as downstream or rear oxygen sensors. 

OBD-II codes will refer to a sensor by a bank and sensor number. Sensor 1 is a term used for an upstream oxygen sensor; sensor 2 is a term used for a downstream oxygen sensor. Understanding bank location is actually quite simple as long as you know where cylinder 1 is. Vehicles that have opposing cylinders (such as V6, V8, H4, etc.) will have cylinder #1 and cylinder #2 on opposite sides. Bank 1 is always the side of the engine where cylinder #1 is, and bank #2 is the side where cylinder #2 is. 

Here is an example of sensor locations on a 2002 Toyota Tundra that has a 4.7L V8 engine (cylinder #1 in this vehicle is on the left or driver side):

• Bank 1 Sensor 1: Driver side upstream
• Bank 1 Sensor 2: Driver side downstream
• Bank 2 Sensor 1: Passenger side upstream
• Bank 2 Sensor 2: Passenger side downstream

Here is an example of sensor locations on a 2002 Nissan Maxima that has a transverse mounted 3.5L V6 engine (cylinder #1 in this vehicle is on the right side or rear bank):

• Bank 1 Sensor 1: Rear bank upstream
• Bank 1 Sensor 2: Rear bank downstream
• Bank 2 Sensor 1: Front bank upstream
• Bank 2 Sensor 2: Front bank downstream

There is only one bank on inline engines, so most of these vehicles will only have a single upstream O2 sensor and a single downstream O2 sensor. Although, some vehicle’s with inline engines may use more than one upstream O2 sensor.

Upstream vs. downstream

The ECU mainly uses information from an upstream O2 Sensor to adjust the fuel trims. If the lambda sensor is not working correctly, the ECU cannot achieve the proper air / fuel ratio needed. This will likely result in poor fuel economy and will eventually lead to damage to the catalytic converter. For this reason, it is extremely important to replace a failed automotive oxygen sensor as soon as possible.

The main purpose of a downstream O2 sensor is to monitor the efficiency of the catalytic converter. The ECU compares the information provided by the downstream sensor to the information received from the upstream O2 sensor. If these readings do not fall within the preset perimeters within the ECU, a catalytic efficiency code will appear. This code does not necessarily mean the catalytic converter needs to be replaced; it simply means the converter is unable to do its job properly. There may be other factors involved contributing to this code as well. When a downstream oxygen sensor fails, the ECU is not able to receive this information.

Heated vs. non-heated

An oxygen sensor needs to be heated to a temperature of at least 650 degrees Fahrenheit to function. The first sensors used on vehicles were 1 or 2 wire non-heated sensors, which use the heat from the exhaust to reach these temperatures. The down side to this is that it takes some time for the engine to generate this exhaust heat. Heated oxygen sensors were developed to overcome this issue. These O2 sensors incorporate a small heating element within the sensor which heats the sensor to minimum operating temperature much faster. This allows the sensor to operate well before the engine reaches operating temperature. Heated oxygen sensors will have at least three wires, and are now the industry standard.

Narrowband vs. wideband

Narrowband oxygen sensors are the most common type of sensor you will find on a vehicle. The first O2 sensors used on vehicles were narrowband, and they are still widely used today. They are a simple metering device, however they are limited. A narrowband sensor can only tell the computer if the engine is running rich (too much fuel in the mixture), lean (not enough fuel), or stiochiometric (14.7:1 air / fuel ratio for gasoline engines). It cannot however tell the computer exactly how rich or how lean the mixture is. This is where wideband oxygen sensors come into play.

Wideband oxygen sensors (also known as “air / fuel ratio sensors” (AFR or AF), or universal exhaust gas oxygen sensors" (UEGO) started appearing in vehicles in the mid 1990s. This type of O2 sensor has the ability to determine an accurate measurement of the air / fuel ratio. By utilizing this type of sensor, the ECU can precisely control the fuel mixture resulting in better performance, fuel economy, and lower emissions. Many modern vehicles today use a wideband upstream oxygen sensor.

Why do oxygen sensors fail?

The exhaust stream of a vehicle is a harsh environment; the sensor is exposed to a variety of possible containments such as oil, coolant, poor fuel mixture, and extreme heat. Any of these can shorten the life expectancy of an O2 sensor, which in ideal circumstances may operate correctly for 80,000 miles or more.

In addition to failing a state emissions test, symptoms of a bad automotive oxygen sensor include poor gas mileage, rough engine idle, and engine surging. A malfunctioning oxygen sensor will also often cause the check engine light to come on, or an On Board Diagnosis indication. Common OBD-II Check Engine Codes related to oxygen sensors include:

P0130 - Bank 1 Sensor 1 Circuit Malfunction
P0131 - Bank 1 Sensor 1 Circuit Low Voltage
P0132 - Bank 1 Sensor 1 No Activity
P0133 - Bank 1 Sensor 1 Slow Resistance
P0134 - Bank 1 Sensor 1 Sensor Circuit No Activity
P0135 - Bank 1 Sensor 1 Sensor Heater Circuit Malfunction
P0136 - Bank 1 Sensor 2 Sensor Circuit Malfunction
P0137 - Bank 1 Sensor 2 Sensor Circuit Malfunction
P0138 - Bank 1 Sensor 2 Sensor Circuit High Voltage
P0139 - Bank 1 Sensor 2 Sensor Circuit Slow Response
P0140 - Bank 1 Sensor 1 Circuit No Activity
P0141 - Bank 1 Sensor 2 Sensor Heater Circuit Malfunction
P0150 - Bank 2 Sensor 1 Sensor Circuit Malfunction
P0151 - Bank 2 Sensor 1 Circuit Low Voltage
P0152 - Bank 2 Sensor 1 Circuit High Voltage
P0153 - Bank 2 Sensor 1 Sensor Circuit Slow Response
P0154 - Bank 2 Sensor 1 No Activity
P0155 - Bank 2 Sensor 1 Sensor Heater Circuit Malfunction
P0156 - Bank 2 Sensor 2 Circuit Malfunction
P0157 - Bank 2 Sensor 2 Circuit Low Voltage
P0158 - Bank 2 Sensor 2 Circuit High Voltage
P0159 - Bank 2 Sensor 2 Sensor Circuit Slow Response
P0160 - Bank 2 Sensor 2 No Activity
P0161 - Bank 2 Sensor 2 Sensor Heater Circuit Malfunction

For an even more in-depth explanation of automotive oxygen sensors, including how many you should have, what else can go wrong, and more, check out our oxygen sensors guide.

Can I replace the oxygen sensor myself?

If your car or truck’s oxygen sensor has malfunctioned, then the vehicle will not be able to determine the air / fuel ratio, and the computer will not be able to make the adjustments needed to ensure the ideal ratio needed. Your car or truck will then begin to run poorly. If your vehicle’s oxygen sensor has failed, it is imperative that you replace it as soon as possible.

The oxygen sensor can be replaced by an experienced do-it-yourselfer. As stated above, the repair will vary depending on the year, make, and model. For an upsteam sensor, the process usually requires the removal of the air intake hose, which can be removed by loosening the hose clamps with a flat blade screwdriver, or the engine cover, which can usually be removed with a socket and ratchet. You may or may not have to remove the heatshield depending on its location, which can make the reinstallation process go a lot more smoothly.

For the downstream sensor, it can typically be removed by raising and securing the vehicle on jack and jack stands. Underneath the vehicle you can disconnect the oxygen sensor wiring harness. To remove the O2 sensor, you will need a special O2 sensor socket. Sometimes, if the O2 sensor will not come off normally, you can cut the wires with wire cutters and remove it with a 7/8 inch socket and ratchet. To install the new sensor, twist the sensor into place and tighten it with an O2 sensor socket and ratchet. Then reconnect the O2 sensor wiring harness. If you removed an engine cover or air intake hose to access the upstream sensor, be sure to reinstall them.