What does an oxygen sensor do and I how can I keep it from causing a smog test failure again?

Dear Car Talk

Dear Car Talk | Jan 01, 1997

Dear Tom and Ray:

Please tell me what an oxygen sensor is. I failed my smog test last month, apparently because of a faulty oxygen sensor. They termed me a "gross polluter." Can you imagine? My car is 1986 LeBaron with only 48,000 miles on it. I finally passed the test, but only after paying $100 for three smog tests and $118 for the oxygen sensor. Is there anything I can do to prevent this from happening again next year? -- Shirley

TOM: Gee, Shirley, if my brother ever referred to me in terms like "gross polluter," that would be a step toward civility in our family!

RAY: The oxygen sensor is an emissions-control device that measures the amount of oxygen in the car's exhaust, and feeds that information back to the car's computer. The computer then uses that information to instantaneously figure out whether the engine is getting the right mixture of air and fuel.

TOM: If the oxygen sensor detects too much oxygen in the exhaust, the computer knows the mixture is running too lean (too much air, not enough fuel). A lean-running engine can produce increased nitrous oxide emissions, so the computer adds more fuel to compensate.

RAY: If the sensor detects too little oxygen in the exhaust, the computer knows the mixture is too rich (too much fuel, not enough air). Rich mixtures increase hydrocarbon emissions, so the computer makes the mixture leaner. And these adjustments take place continuously -- many times a minute.

TOM: And there's nothing you need to do to avoid this happening next year. Your oxygen sensor simply wore out after 10 years. And the way you drive, you should get another 10 years out of this one. The only thing I'd worry about for next year is the catalytic converter.

RAY: Right. If your oxygen sensor was broken for a long time, the mixture may have been overly rich and may have overworked your catalytic converter. And unless you're overly rich, you're going to find buying a new converter a shocking experience. So I Hope you caught it early, Shirley!

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