What do those pricey oxygen sensors do anyway?

Dear Car Talk

Dear Car Talk | Jan 01, 2001

Dear Tom and Ray:

We own a 1996 VW Golf with only 28,000 miles on it. The engine warning light came on and stayed on. We took it to our dealer, who said it was the front oxygen sensor. It cost $182 to fix. A week later, the engine light came on again and stayed on. This time, the dealer said the rear oxygen sensor is no good. Now it will be $360, plus tax. What are these sensors? What do they do? That's a lot of money for a little four-cylinder car with 28,000 miles on it. -- Bobie

RAY: You think THAT's an expensive part for such a little car, Bobie. Wait till your $700 air-flow sensor goes on the fritz!

TOM: The oxygen sensors are part of the car's emission-control system, Bobie. The sensors measure the amount of oxygen in your exhaust. And that information tells the car's computer whether the engine is getting the right mixture of air and fuel.

RAY: If there's too little oxygen in the exhaust, the computer knows that the fuel injectors are sending in too much fuel. If there's too much oxygen in the exhaust, not enough fuel is going in.

TOM: And based on those readings, the computer continuously adjusts the pulses of the fuel injectors so that the mixture is just right and emissions are minimized.

RAY: Your car is a little too old for this to be covered by warranty. But we want our other readers to know that for cars built in 1995 or later, the Environmental Protection Agency requires car makers to warranty every single emissions-related component in the car for two years or 24,000 miles. And that includes diagnosis, parts and labor.

TOM: In addition to that, for a full eight years or 80,000 miles, car makers have to warranty the "primary" emissions components, which are defined as the catalytic converter, the computer (ECU) and the on-board diagnostic device -- all expensive parts.

RAY: So if any of you readers think this federally mandated warranty might apply to you, you can get all of the details in the EPA pamphlet regarding emissions warranties by calling the EPA library at (734) 214-4311. Or you can get it on the Web at http://www.epa.gov/oms/consumer/warr94fs.txt. That explains your rights and the manufacturer's responsibilities.

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