How do oxygen sensors work?

Dear Car Talk

Dear Car Talk | May 01, 1993

Dear Tom and Ray:

I have a question about the oxygen sensor on my 1981 Toyota Corolla. How does it work and what is its purpose?

TOM: That's a good question, John. The oxygen sensor is part of the emission control system.

RAY: Isn't that in Houston? Where NASA talks to the space ships?

TOM: That's "mission control," you knucklehead. "Emission control" is a system that monitors how your engine is running by analyzing the exhaust. It's kind of like what your doctor is doing when he or she gives you a little cup and asks you for a "sample."

RAY: The oxygen sensor sits in the exhaust system, just in front of the catalytic converter. And several times a second, it measures the amount of oxygen in the exhaust.

TOM: What this tells you is whether the right amount of fuel and air is going into the cylinders. If there's too much oxygen in the exhaust, that means there's not enough fuel. And if there's too little oxygen left over, that means the mixture's too rich--that there's too much fuel.

RAY: On modern cars, the oxygen sensor is connected to the computer. And the computer instantaneously adjusts the fuel/air mixture to make sure it's always perfect. That improves fuel economy, cuts down on pollution, and most importantly, extends the life of the catalytic converter, which the manufacturer is obligated to warrant for 50,000 miles.

TOM: But your car is pre-computer, John. So instead of adjusting the mixture automatically, your oxygen sensor turns on a light on your dashboard. That signals you to take the car to your mechanic to have the mixture and sensor checked out. It's a little more cumbersome than having the computer do it several times a second, but that was as good as it got in 1981.

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