If your car was built since the mid-1980s, chances are it has an oxygen sensor in its exhaust system. An oxygen sensor measures the oxygen content of a car's exhaust gases, to help the engine-control computer regulate the mixture of air and fuel burned in the engine to provide an optimal balance of power, economy and clean exhaust.
It requires periodic replacement, generally every 30,000 to 50,000 miles, as mentioned in the owner's manual. Oxygen sensors are relatively simple to replace with most models costing $50 to $100. While on-time oxygen sensor replacement is vital, many motorists skip it when they should because they didn't see noticeable difference in the way a car drives when the sensor stops functioning normally. When drivers don't notice anything wrong until drive-ability suffers, in the meantime, dirty exhaust is already creating an environmental problem.
Because oxygen sensor replacement is important, some car makers place a glowing red light on the instrument panel as the sensor replacement interval approaches, and it keeps glowing until the car is serviced (well, while the ignition is on). Others cars' instrument panels include a series of different dash lights, each one for a component of the engine control system, including the oxygen sensor. In this case, you still must replace your oxygen sensor at the mileage interval specified by the car maker, though the lights won't come on at replacement time, only at time of problem. Then it's up to the service technician to 'interrogate' your car's computer to find out which system or systems aren't working correctly. Further tests may then be needed to find out what's wrong with the affected system or systems. He may end up spending more time investigating a problem, not knowing he can rule out the (recently replaced) oxygen sensor.