Oxygen sensors have two basic functions: upstream sensors close to the manifold meter the amount of oxygen and burned fuel mixture in the exhaust; downstream sensors located near the catalytic converter monitor the catalyst efficiency. If too much fuel or too much oxygen is read by the sensor, it transmits the information via radio signal to the PCM, which then makes adjustments in the engine to correct the problem. Oxygen sensors wear out after a while and need to be replaced during regular maintenance schedule intervals.
Finding the Right Sensor
The hardest part of replacing an oxygen sensor is locating the correct one. Vehicles nowadays have at least two, but some can have up to four sensors. When the sensor fails to transmit or a noticeable problem in the fuel mixture is detected, the PCM will trigger the "check engine" light to alert the car owner that there is a problem. All vehicles built after 1996 have universal PCM plug-in sources called "DLCs," or "data link connectors." A scan tool or code reader will allow you to find out the problem or narrow it down by plugging it in to the DLC and then reading the code. But what if it says something like, "circuit failure sensor 1 bank 2?" Banks and sensor numbers are used to help identify the location of them in the exhaust system. "Sensor 1" indicates a forward or upstream sensor towards the manifold; "banks" indicate the split in the exhaust system, such as a Y-pipe coming off the engine or a genuine dual exhaust system. Bank 1 is most often located on the same side of the vehicle--left or right--as the number one cylinder. So sensor 1 bank 2 would be located upstream near the manifold on the opposite side of the number one cylinder.
Access to some of the sensors nowadays is somewhat challenging. Most vehicle manufacturers place the oxygen sensors in conspicuous places that can easily be accessed. Others may make you wonder what they were thinking when they designed the exhaust system. There are tools to help make it easier for you and a trick or two that might work. Once you've located the correct sensor or sensors, unplug the sensor from the wire harness.
Oxygen sensor wrenches or sockets are available in most parts stores nowadays. Although they make the job a whole lot easier, they're not magic wands. Limited access is still going to challenge you, and corrosion on the old sensor and exhaust pipe may be another obstacle. Remove the sensor once you've gotten the tool onto it by turning the tool counterclockwise. A degree of force will be required, but if it doesn't feel like it's going to budge, heating up the portion of the exhaust pipe that surrounds the threads of the oxygen sensor might help. This can be done with a portable propane torch. Don't heat the sensor as well. You're goal is to expand the exhaust pipe and not the sensor. Being careful not to burn yourself, retry removing the sensor with the wrench or socket until you're successful. If you don't have a torch, try running the car long enough to heat the exhaust system up for a while and then liberally spray lubrication oil on the sensor.
Replacement sensors range in price from $50 to $120 a piece. Bosch makes a decent direct-fit aftermarket sensor made specifically for most vehicles at an affordable price. The dealership will sell direct-fit original equipment sensors, but will be on the high end of the price range. You may discover in your hunt for the best price that a universal sensor is available for many cars. This means the sensor can be wired in and used in more than one application non-specifically. Although it's cheaper and sounds like a good deal, keep in mind that in addition to removing and replacing the oxygen sensor, you're going to have to cut and splice wires--that most likely will not color-code the same way on both ends. For a few dollars more, your money will be well spent for a name-brand direct-fit oxygen sensor made specifically for your vehicle.
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