Taking the mystery out of Oxygen Sensors.

Many automobile owners and enthusiasts have heard of oxygen sensors, and know that their car has one or more of them. But how many people really understand their purpose and importance to the vehicle’s engine? 

New Oxygen Sensor

New Oxygen Sensor


Modern fuel injected engines depend on a large number of factors to keep the engine running smooth and efficient.  One of the most important factors in this complicated process comes from the oxygen sensors. The oxygen sensors send data to the computer to help it determine whether the engine is running rich (too much fuel) or lean (too little fuel) and the computer can then make the proper adjustments to the fuel ratio.

Oxygen sensors (o2 Sensors) have been used in almost every car since the early 1980’s. They have progressed technologically to the efficient and reliable state that they are today. Oxygen sensors depend on being hot in order to work properly, the original sensors would not work until the heat from the exhaust actually heated them up enough to start working. During that heat up time, the computer could not use the oxygen sensor data to help determine the fuel ratio. This would cause early fuel injected engines to be very inefficient until the exhaust has heated up the oxygen sensor enough to allow it to start working. Modern sensors actually have a built in heater which very quickly heats the sensing element inside the oxygen sensor so that it can begin sending data almost immediately to the computer after start-up.

For many people the terminology used to explain oxygen sensor locations can be intimidating but it’s actually quite simple. They are usually listed one of two different ways, the more common way is, for example, Upstream bank 1, or Downstream bank 2 etc. This odd sounding terminology is actually quite simple to understand. The ‘Bank’ is simply a way of explaining which side of the engine the sensor is on. Bank 1 is always the side that has cylinder number 1 on it, you may have to reference a firing order diagram to determine where cylinder number one is. Bank 2 is always the side opposite of bank 1. Upstream and Downstream are even easier. Upstream simply means before the catalytic converter (closer to the engine), while downstream means after the converter (closer to the tailpipe). So here is an example, if you want to replace the Upstream bank 2 sensor, you simply find the number one cylinder, go to the opposite side of the engine, and the correct sensor is the one in the exhaust closest to the engine. But wait, there’s more! Another terminology common to Oxygen Sensors looks like so: B1S2, B2S1 or B1S1. This is actually easy to understand as well, the B# stands for Bank # , and the S# stands for Sensor #. the Bank is the same as the previous example, and for the sensor number, S1 means upstream (before the converter), S2 means downstream (after the converter). So if you see B1S2 that means the sensor on Bank 1 After the Catalytic Converter.

So what’s the difference between upstream and downstream oxygen sensors? Actually quite a bit. They both have unique functions, the upstream sensors’ main function is to monitor the air fuel ratio coming directly out of the engine, and use that data to help the engine perform properly. The downstream sensors, however, have a completely different use. Their primary task is to monitor how effectively the catalytic converter(s) are working. When a catalytic converter is working properly, the downstream oxygen sensor will output a relatively steady voltage of about .5 volts (after everything is at optimal temperature). If the converter is going bad, the voltage coming from the downstream sensor will be almost identical to the voltage coming from the upstream, which fluctuates between .2 and .8 volts while the engine is running.

Oxygen sensors fail for a number of reasons, one of the most common failures in modern sensors is the built in heater failing. When the heater fails, the sensor becomes like the older ones and depends on the exhaust to heat it up before it begins working which again causes the engine to run inefficiently until the sensor heats up fully. All cars after 1996 are designed to monitor the heaters inside the oxygen sensor, and will turn on your check engine light if they detect a problem with any of them. Another common failure of o2 sensors is caused from the buildup of carbon and foreign material on the sensor itself. Over time this can cause the sensor to become “sluggish” in it’s response time, which can can also affect performance and fuel economy. The computer will usually turn on the check engine light when it detects that the sensor is responding sluggishly. There is no way to clean or repair the sensor once it becomes sluggish and must therefore be replaced.

After years of usage, oxygen sensors can degrade in performance  yet not cause your check engine light to come on. If the computer “assumes” the sensors are working properly, but they are really sending inacurate data, the computer will make adjustments to the fuel ratio that should not be made. This can also cause numerous problems, from decreased fuel economy, to poor running condition.

Oxygen sensors are subjected to severe conditions throughout their life in your vehicle.  Over time the constant exposure to temperatures as high as 1,000 Degrees Fahrenheit as well as carbon buildup on the sensor, will cause the sensor to be slow to respond and less accurate. This can cause a reduction in performance, and a drastic drop in fuel efficiency.

Old Oxygen Sensor

 Old worn oxygen sensor.

                Replace your Oxygen Sensors every 60,000 – 100,000 miles to help keep your engine running in peak condition and prevent future problems. You can order brand new oxygen sensors for your car here: www.autopartsdirect2you.com


– Joe Stepnicka


-Duplication of part or all of this article is strictly prohibited without the author’s written permission.


About Josh

I help manage the site here at Auto Repair Instructions. If you are having problems, then I am the one to contact.

Posted on November 28, 2009, in Uncategorized. Bookmark the permalink. 5 Comments.

  1. This is a great site. Good sparkling interface and very informative articles. I will be coming back soon, thanks for the great article. In some areas, there are waiting lists.

  2. Hey thanks for this article

  3. You kind of glossed over the difference between upstream and downstream O2 sensors. You listed the voltage output, but that is a function of the oxygen present and not a difference between the sensors construction. My question would be if the wiring connectors are identical could a downstream O2 sensor be put into an upstream location? What fault would occur?

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