Gas engines work by combusting a mixture of air and fuel in the engine cylinders. The efficiency of the engine depends on this mixture burning at a precise ratio. If the ratio is off, it can cause harmful emissions and may damage your engine. An oxygen sensor measures oxygen levels in your exhaust fumes to gauge engine efficiency. The mixture is classified as “rich” if there is not enough oxygen and “lean” if there is too much.

Oxygen sensors are typically mounted directly onto the exhaust pipe. One part of the sensor is located within the hot exhaust flow, while the other is in contact with the outside air. The difference between the two readings generates an electrical signal that tells your car’s Powertrain Control Module how much to alter the air-fuel mixture.

Standard vehicles come with at least two different oxygen sensors. The second sensor measures oxygen in the exhaust flow after the fumes have passed through the catalytic converter. A catalytic converter is responsible for converting some unwanted exhaust gasses to help reduce the toxic emissions released. 


Oxygen sensors are not only essential for engine efficiency but also for keeping exhaust emissions at acceptable levels. Some states require that vehicles undergo emissions testing every couple of years (it varies by state), and a malfunctioning oxygen sensor will likely cause your vehicle to fail an emissions test.

Here are a few signs an oxygen sensor isn’t working properly and may need to be replaced:

  • The check engine light is illuminated
  • Your gas mileage has dropped
  • There’s a strong fuel or rotten egg smell when the car is running (caused by unburnt fuel left in the engine)
  • There’s black smoke coming from the tailpipe
  • Poor engine performance, such as poor acceleration, engine misfires and stalling

Check out our video to learn more about oxygen sensors, when they may need to be replaced and what to expect during a service appointment.


Oxygen sensors generally last between 60,000 and 90,000 miles, but there are always exceptions. While they don’t need regular maintenance, if you notice any of the above signs, bring your car to Firestone Complete Auto Care so an expert can inspect the sensors and recommend a replacement if needed. Since most vehicles have multiple sensors, it’s important to have them all checked to ensure you’re leaving with all the sensors in working order.

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