I would say the O2 sensor is the most important sensor in the car... It controls whatthe ECM does. I got this off another site
Modern, sophisticated cars don't require as much servicing as older models. Still, if your car was built since the mid-1980s, chances are it has an oxygen sensor in its exhaust system. You may never have encountered an oxygen sensor before, but it's a part which requires periodic replacement as part of your routine servicing. Otherwise, you could experience poor vehicle performance on the highway.
An oxygen sensor is designed to measure the oxygen content of a car's exhaust gases. This information helps 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.
Oxygen sensors are relatively simple to replace with most models costing $50 to $100. Replacement intervals range from 30,000 to 50,000 miles, and are generally listed in the owner's manual.
While on-time oxygen sensor replacement is vital, many motorists don't have the job done when they should because there isn't a noticeable difference in the way a car drives when the sensor stops functioning normally. Some motorists may even skip oxygen sensor servicing on purpose, figuring there's no need to do anything until the car starts to "act up".
Either way, they're making an expensive mistake. An engine may use too much fuel when the oxygen sensor isn't working. This doesn't just hurt economically, it also creates dirty exhaust, polluting the air.
An alert motorist may notice worsening fuel mileage or driveability problems and have the sensor changed. But he adds that since most drivers don't notice anything wrong until driveability suffers, dirty exhaust may already be coming from the vehicle creating an environmental problem.
Seeing The Light
Because oxygen sensor replacement is important, many car makers include hard-to-ignore reminders, like a glowing red light on the instrument panel as the sensor replacement interval approaches. The light keeps glowing (while the ignition is on) until the car is serviced.
Another Indicator
Some cars' instrument panels include a different dash light meant to indicate problems with components of the engine control system, including the oxygen sensor and other items. Typically, these lights are labeled "service engine soon," "check engine," "power loss," or something similar. If your car uses this system (check your owner's manual), you still must replace your oxygen sensor at the mileage interval specified by the car maker.
The light won't come on simply to let you know it's time to replace your oxygen sensor, it's up to you to keep track of when the sensor needs servicing.
Since its glowing may indicate one or more of a number of problems, a service technician will have 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."
If you're a serious do-it-yourselfer (DIYer), consult the correct shop manual for your car if this light comes on. In many cases, the manual will explain step-by-step procedures to make the light signal (using semaphore-type flashing codes) where the problems lie.
Care About Your Car
Routine auto servicing is different than it was a few years ago. Though in most respects it's easier, it still comes down to paying a little now or a lot later.
Today's high-tech cars don't need constant attention, but there hasn't been a car invented yet that can take care of itself. Make a point of learning your new car's service requirements. As with the oxygen sensor, something you've never heard of before may need occasional attention.
