The Ultimate Guide to The Oxygen Sensor for Dummies

Oxygen sensors are utilized by vehicles to help control emissions and make certain the exhaust system is running efficiently. In most newer vehicles equipped with 4 cylinder engines there are two oxygen sensors, one before the catalytic converter and one after. Some V6 and V8 vehicles have even more oxygen sensors to help monitor their complex systems. The primary purpose of the oxygen sensor is to reduce automobile emissions and help to keep the environment clean.

History of the Oxygen Sensor

The Robert Bosch Company developed the first oxygen sensor for the 1976 Volvo. The first sensors produced relied on the heat from the exhaust to heat up to operating temperature. This created an issue with performance because it took almost a minute for the sensors begin to respond.

The oxygen sensor was first made mandatory equipment on vehicles in 1980 when the state of California saw the potential for the oxygen sensor to significantly reduce emissions. By the mid-1990s, every state in the United States had laws making oxygen sensors mandatory.

Over the years, the oxygen sensor has taken on a variety of names that all refer to the same sensor. Some of the other names an oxygen sensor is known by throughout the auto industry include the lambda sensor (primarily in the UK), lambda probe, exhaust gas oxygen (EGO) sensor, heated exhaust gas oxygen (HEGO) sensor, planar sensor, and O2 sensor.

Lambda Sensor

[ Photo by Martin Olsson (Wikimedia Commons) ]

How Oxygen Sensors Have Evolved

When it was first introduced, the oxygen sensor would monitor the exhaust on a vehicle and maintain the proper mixture of air and fuel. Advancements in fuel delivery from the use of a carburetor vs. fuel injection helped with the fine adjustments needed to be made with the input given from the sensor to the vehicle's computer system. The balance of air and fuel was delivered to the engine much more efficiently improving on fuel consumption and the environmental impact of exhaust emissions.

The original O2 sensors were single-wire devices that did not have heaters built into them. These sensors used the metal bar method for heating up, but they could not maintain their heat when the car idled for an extended period of time. To combat this problem, multi-wire oxygen sensors were developed that included heaters to help keep the sensors operating properly at all times. Since the sensor must be approximately 600 degrees or higher before it will begin to function a warming rod was added to the design helping move heat from the engine and get the sensor operating quicker. This improvement to the system design is still used today.

By 1996, oxygen sensors were standard equipment on all vehicles and more sensors were added to help monitor the efficiency of the fuel and exhaust systems. Around this time, the On-Board Diagnostics II system was also introduced to add an extra layer of computerized monitoring for more complex fuel systems. As O2 sensors started to become more important to the overall efficiency of auto engines, the OBII system evolved to become a very precise and reliable monitoring system.

Since dual-exhaust systems have two catalytic converters, they also have double the amount of oxygen sensors. When you look at error codes for O2 sensors, you will see indicators such as "bank 1 sensor 1" or "bank 2 sensor 2." In single-exhaust systems, you will usually only get a "bank 1 sensor 1" error. Bank 1 means the first bank of engine cylinders, and bank 2 means the second. Sensor 1 is usually the sensor closest to the engine, and sensor 2 is the sensor located on the other side of the catalytic converter.

Wideband Zirconia Sensor

[ Photo by Michael Handrich (Wikimedia Commons) ]

How Do They Work

The O2 sensor actually generates electricity based on the output of the exhaust system. The computer then senses this variance in voltage making adjustments to the fuel mixture. The voltage produced ranges from 0.9 down to 0.1 volts telling the computer the mixture is either too lean or too rich. The higher the voltage is, the richer the exhaust from the engine is and this information allows the car to lean the mixture out.

Sometimes the voltage output from the sensor can fail or the variance in voltage can become sluggish, not responding fast enough for the computer. When the sensor fails the computer is unable to perform key adjustments to allow the engine to perform efficiently and will cause a check engine light to appear. Identifying which sensor has failed can be tricky and most times it requires the use of advanced scanning tools that connect to the vehicle's computer system.

Vehicles with multiple sensors are monitoring the performance of the catalytic converter as well. If a sensor is located before the catalytic converter its job is to adjust the mixture of the fuel whereas the sensor after is monitoring the catalytic converters performance and efficiency.

Can I Replace My Oxygen Sensor?

Oxygen sensors need to be replaced every so often due to wear and tear, and most people rely on a professional mechanic to do the job. Newer sensors can last for many miles and preventatively changing them is not always necessary. However, if your exhaust system is showing signs of rust or is susceptible premature rusting and failure, it may be a good idea to consult with a technician to see if preventative replacement might be a good idea even if it is performing correctly.

Scanners to determine if a sensor is bad or not have become affordable and are easy to plugin to your cars data link connector. It's a good idea to have a repair manual for your car in case you need further information regarding your specific vehicle's data link connector or further requirements for removal and replacement. Once you have determined that an oxygen sensor has failed, you can refer the repair manual or purchase the replacement sensor and size it to a socket or wrench depending on the location and accessibility of the sensor. If you do decide to purchase an oxygen sensor wrench and replace the sensor on your own, it can be favorable to remove it while the exhaust is warm or hot however can be very dangerous and therefore it is recommended to wait until it has cooled.

How Much Is an Oxygen Sensor?

If you elect to replace the sensor yourself you can save money. However, if the repair seems too challenging, it can become quite expensive depending on the vehicle. By taking your car to mechanic you have to take in to account you will be paying a marked up price for the sensor as well as an additional charge of labor to remove and replace the part.

Most mechanics will not allow you to purchase a part from a parts store and have them install it. They rely on both the mark up of the part as well as their labor rate as part of their business income. An oxygen sensor that might cost you $100 if you were to perchance it yourself can range from $125 to $150 after their 25% to 50% markup and labor can range from $85 to $110 per hour. Labor rates are usually higher at a dealership and can be as much as $135 per hour. The good news is that for most vehicles, only an hour or less is required for the repair.

Oxygen sensors tend to be fairly reliable aftermarket but consideration should be made when purchasing them. A universal oxygen sensor might cost lest however it will not come with the factory connector used to plug into the car. If you elect to purchase a universal one, you will have to cut off and reuse the original connector from the faulty sensor. Universal sensors will come with instructions as well as butt splice connectors to reuse the connector however original equipment oxygen sensors are usually the best choice when replacing due to their direct fit and ease of installation.

What Can Happen if I Don't Replace it?

Oxygen sensors rely on electrical currents to monitor and maintain the air/fuel mixture in the car. As the sensor gets older, it starts to lose the ability to properly monitor the mixture, and that allows contaminants to get onto the sensor. At this point, the sensor is essentially blind and will start giving off incorrect electronic signals that will improperly measure the air-fuel mixture.

The most significant issue with a malfunctioning oxygen sensor is that it will affect your car's fuel efficiency. As the sensor continues to fail, your car will burn more fuel than usual. Initially, you may not notice the lack of efficiency in your vehicle's fuel system. If there are no indicator lights alerting you to a potential problem with your O2 sensor, then you may not notice a problem at all. Over time, though, you will notice a drop in your fuel efficiency that should prompt you to either scan your vehicle yourself or have your mechanic look your vehicle over.

Another issue with a failing O2 sensor is that it will force your emissions system to run constantly in what is known as an open loop. If the loop is not closed, then the emissions system cannot regulate your exhaust. In a state vehicle inspection, a damaged O2 sensor will normally cause your vehicle to fail an emissions test.

Most people don't even realize they have an oxygen sensor or two on their car, but the oxygen sensor is one of the most important parts on any vehicle. As emissions systems continue to evolve, so will the role of the oxygen sensor in keeping our atmosphere clean and our vehicles running properly.


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