The symptoms of a bad oxygen sensor or O2 sensor include mileage decrease, trouble code display on on-board diagnostic system and a smog test failure. Among other factors, an illumination of the car's malfunction indicator lamp can be a sign of a faulty oxygen sensor.
An oxygen sensor is a device that regulates the amount of oxygen that mixes with fuel to ensure proper fuel consumption. A malfunction in the oxygen results in an incorrect air-fuel mixture, leading to lower mileage and eventual fuel overconsumption.
When an on-board diagnostic system detects that an oxygen sensor has a problem, it displays a code that corresponds to the problem. Although a more accurate detector of exhaust system problems than other detectives, all the cars manufactured earlier than 1996 lack on-board diagnostic system.
A faulty oxygen sensor results in the emission of an inappropriate amount of carbon monoxide and contributes to a smog test failure to a greater extent. About 50 to 60 percent of smog test failures result from defective oxygen sensors, according to California Air Resource Board and Environmental Protection Agency. To avoid fuel overconsumption that results from a faulty oxygen sensor, the car owner should replace the sensor immediately when the symptoms appear.