What's An Oxygen Sensor, and Why Is It Important?

Modern cars with internal combustion engines have oxygen sensors in the exhaust system. An oxygen sensor is a tube-shaped device normally mounted inside the exhaust manifold. They also may be found mounted just in front of the catalytic converters. Vehicles typically have between one and four oxygen sensors.

Toyota O2 sensor

The amount of sensors your Toyota vehicle has depends on the model. The newer your car and the more powerful your engine, the more oxygen sensors it’s likely to have. The service manual for your vehicle should tell you how many oxygen sensors it has.

How an Oxygen Sensor Works

An oxygen sensor (aka O2 sensor or lambda sensor) is designed to detect the amount of oxygen in the exhaust. It does this up to 7 times per second. It compares the amount of oxygen in the exhaust fumes to the amount of oxygen outside the vehicle. The O2 sensor then sends the data to the ECU which adjusts the air/fuel mixture accordingly.

This process exists to help your Toyota engine maintain the ideal fuel mixture at all times. The ideal fuel mixture varies depending on many factors, including:

  • Engine temperature.
  • Engine load.
  • Air temperature.
  • Throttle position.

Read more about how an oxygen sensor works here.

Symptoms of a Failing Oxygen Sensor

O2 sensor locationImage Credit: wes shaefer

When an oxygen sensor fails, your engine will either run too rich or too lean. This leads to a bunch of symptoms cropping up.

Running rich means there’s too much fuel in the air/fuel mixture. Symptoms of a rich fuel mixture include:

  • Reduced fuel economy
  • Reduced power
  • Sluggish acceleration
  • Visible soot in the ends of the exhaust pipes
  • Uneven running
  • Strong gasoline smell while idling

A lean mixture is the opposite of a rich mixture. It means there’s too much air in the air/fuel mixture. The ECU reacts to a lean mixture by retarding the engine’s timing to prevent overheating or even detonation. Running lean shortens spark plug life, reduces fuel economy, and decreases the engine’s overall power. Aside from the check engine light coming on, the following symptoms are indicators of your engine running too lean:

  • Difficulty starting in cold weather
  • Rough, uneven idle
  • Decreased throttle response
  • Surging when not prompted
  • Engine running hotter than normal

Diagnosing and Replacing a Failing Oxygen Sensor

The above symptoms are good indicators that there’s a bad oxygen sensor in your exhaust system. But, you want to be 100% sure that's the case before buying a replacement oxygen sensor. You can either have a Toyota dealership do a diagnostic check or diagnose the problem yourself. If you indeed have a bad oxygen sensor, we have good news: it’s pretty easy to replace.

We’ll post two tutorials in the upcoming weeks: diagnosing a bad oxygen sensor and replacing an oxygen sensor. Stay tuned!

Written by Jason Lancaster