How O2 sensors work

O2 sensors are also known as oxygen sensors or lambda sensors. Where are O2 sensors located? They're positioned within a vehicle's exhaust system and are used to check that the air/fuel mixture being fed into the engine is correct. They also check that the catalytic converter is operating efficiently.

Upstream oxygen sensors generate a small variable voltage signal in proportion to the difference between the oxygen content in the exhaust stream and the oxygen content in the ambient air. The Powertrain Control Module (PCM), which is the brain of the emissions system, uses this information to maintain the proper air/fuel ratio.

Downstream oxygen sensors (sometimes called ‘catalyst monitor’ sensors) are so-named because they are ‘downstream’ from the catalytic converter in the exhaust system. Downstream sensors also generate a small variable voltage signal to let the PCM know if the catalytic converter is operating at maximum efficiency. If not, an error code is set and the MIL (more commonly known as a ‘Check Engine’ light) is illuminated.

Typically there are multiple oxygen sensors - two for each cylinder bank. Four-cylinder engines have two sensors, while V6 and V8 engines have four sensors. The upstream oxygen sensor(s) are located below the exhaust manifold flanges and above the catalytic converter. The downstream oxygen sensor(s) are located in the exhaust pipe, behind the catalytic converter.

When an O2 sensor goes bad

What happens when an O2 sensor is bad? The vehicle's PCM will turn on the ‘Check Engine’ light (MIL) when there's a problem in any of the O2 sensor circuits. The associated trouble code stored in the PCM will indicate which O2 sensor circuit is at fault. The problem is not always the sensor itself. Sometimes the problem is a loose or damaged wire to the sensor.

Failed O2 sensor symptoms also include driveability problems such as a loss of power when accelerating, a fluctuating idle and poor fuel economy. These symptoms are caused by the PCM constantly trying to adjust the air/fuel mixture because it is receiving false information, or no information at all, from the sensors.

Why you should change your O2 sensor

When the O2 sensors are not working properly, fuel economy can drop substantially, which means increased fuel costs.

If a failed sensor is not fixed, the other sensors in the system, as well as the catalytic converter, can be damaged.

Having your vehicle running with excessive emissions can damage the environment.

How to change your oxygen sensor

Watch this video to see how it's done. Find the full step-by-step task for your model.

Here is an example clip showing how to change a car's oxygen, or lambda, sensor. Find the full step-by-step task for your model below. A very brief summary of the task:

Here are a few precautions before servicing an oxygen sensor:

a)  Oxygen sensors have a permanently attached pigtail and electrical connector that can’t be removed from the sensor. Damage to or removal of the pigtail or the electrical connector will ruin the sensor.

b)  Keep grease, dirt and other contaminants away from the electrical connector and the louvered end of the sensor.

c)  Do not use cleaning solvents of any kind on oxygen sensors.

d)  Do not drop or roughly handle oxygen sensors.

e)  Be sure to install the silicone boot in the correct position to prevent the boot from melting and to allow the sensor to operate properly.

Because the O2 sensors are installed in the exhaust manifold or pipe, both of which contract when cool, an O2 sensor might be very difficult to loosen when the engine is cold. Rather than risk damage to the sensor or its mounting threads, start and run the engine for a minute or two, then shut it off. Be careful not to burn yourself during the following procedure:

  1. Disconnect the cable from the negative battery terminal.
  2. Raise the vehicle and place it securely on jackstands.
  3. If necessary, remove the exhaust heat shield, which is usually retained by bolts.
  4. Disconnect the oxygen sensor electrical connector. Using an oxygen sensor (slotted) socket, remove the oxygen sensor. If the sensor is difficult to loosen, spray some penetrating oil onto the sensor threads and allow it to soak in.
  5. Apply a light coating of anti-seize compound to the threads of the new oxygen sensor to facilitate future removal. Do not get any anti-seize on the ceramic part of the sensor that goes into the pipe. Some new sensors come with anti-seize compound already on the threads.
  6. Be sure to tighten the sensor securely.

Tools you will need

  • Floor jack (if necessary). Not your car’s emergency jack
  • Jackstands
  • Socket set
  • O2 (slotted) socket
  • Pliers
  • Needle-nose pliers
  • Flat-bladed screwdriver
  • Phillips screwdriver

Parts that you may need

  • O2 sensor(s)

How much do new O2 sensors cost?

O2 sensors
£75-£200

Garage fee savings
£50-£150