Tech tip - Oxygen Sensors

Here is another tech tip/info article on the often misunderstood Oxygen Sensor.

Well as you may have figured, an Oxygen sensor senses oxygen right? Well, yes but more accurately put, it senses the lack of oxygen!

It is placed in the vehicle's exhaust stream and it detects and measures how much oxygen is in the exhaust gas. A quick lesson in what is actually in exhaust gas - you probably hear a lot about Carbon Dioxide (CO2) emissions but there is more to exhaust gas than this alone. You also have hydrocarbons (HC), Carbon Monoxide (CO) and Oxygen (O2) as the "big 4", as well as Oxides of Nitrogen (NOx) and other less significant gasses. However, right now we are only concerned with oxygen. Now you might be wondering why it is important for the ECU to know how much oxygen is in the exhaust? Well I'm glad you asked! This is actually a great way of measuring the fuel mixture (how rich or lean) quickly and easily. This is primarily done so that the manufacturers can meet emission standards, but the bonus for you is that is is great for your fuel economy!

Basically, when the conditions are right, the ECU has a look at the O2 sensor and if it is reading "lean" (lots of oxygen content) it slowly richens the mixture until it gets a "rich" (little or no oxygen content) reading and then it slowly leans the mixture off until it gets a "lean" reading and the process then repeats itself over and over again. The ECU is now in "closed loop" mode. When I said slowly before, I meant slow in computer terms, as this "rich/lean" cycle can repeat itself up to 3 or 4 times a second depending on the vehicle and conditions, but is usually maybe 1 or 2 times per second. This ensures that the car runs as close as possible to the ideal mixture, which is 14.7:1 air/fuel ratio (AFR). This is also called Lambda 1.

Now to briefly tell you how this sensor works! It has a zirconia coated ceramic element inside that generates a small voltage when there is a difference in oxygen content across it. Basically, when the amount of oxygen inside the exhaust is different to the amount outside, you get a voltage. This voltage is only one volt, but as long as it changes from zero to one volt the ECU can see it.

Of course there is a lot more to it than this, but this gives you an idea :)

There are also other types of sensors, such as Titania (instead of Zirconia) as well as wide-band sensors. I also have not mentioned the difference between single wire or 2, 3 or 4 wire sensors, perhaps that is for a later article.

Happy motoring!

The Doctor.