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Replace Oxygen Sensor:

An oxygen sensor is a part that monitors the unburned oxygen from the exhaust. It helps monitor how much fuel is burned. A faulty sensor means it’s not providing the right data to the computer and causes a decrease in gas mileage. Most cars have between two and four oxygen sensors and the code you get from the scanner will tell you which one needs replacing.

What causes it: Over time, the sensor gets covered in oil ash and it reduces the sensors ability to change the oxygen and fuel mixture. A faulty sensor not only reduces gas mileage, it also increase emissions.

What you should do: Not replacing a broken oxygen sensor can eventually lead to a busted catalytic convertor which can cost upwards of $2,000. Taking your car into a shop will cost you around $200 depending on the car. However, an oxygen sensor is easy to replace on many cars and is usually detailed in the owner’s manual. If you know where the sensor is, you only have to unclip the old sensor and replace it with a new one. Regardless of how you approach it, you should get this fixed right away.