If you're driving a Daewoo produced after 1980, then you're driving a Daewoo that was manufactured with an oxygen sensor.  The oxygen sensor is part of the emissions control system and is responsible for sending important data to the engine's computer.  In every vehicle, there is a perfect ratio of fuel and air that must be continuous in order for your engine to run efficiently and your emissions to stay as low as possible.  Your oxygen sensor is the reader of this type of data.  It reads the ratio, sends its findings to the engine's computer where the fuel can be adjusted to the amount of air in the fuel/air mixture.  A failing oxygen sensor cannot send this valuable information to the engine's computer.  As a result, your vehicle will run poorly.  You may experience poor gas mileage, hesitation, poor acceleration, surging, and emissions test failure if your oxygen sensor has stopped functioning as well as it should.