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O2 sensor operation
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 6 Apr 2005 06:19:43 Share Post by Email
In Reply to: Re: Controlled fluctuation requires connection, kyle morley, Tue, 5 Apr 2005 15:33:23
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The O2 sensor is like a battery that operates on oxygen and heat. Actually, Oxygen difference. The difference in oxygen between the back and front (probe) of the sensor produce a small voltage. Air comes in the back of the sensor (wire side) and provides the reference. If there is more oxygen at the probe of the sensor than coming in the back, a small voltage is produced. The bigger the difference (less air at the face), the bigger the voltage.

However, this battery doesn't work unless it is very hot - at least 200-300C. At room temperature, it won't produce a voltage even if there is no air at the front.

When you play the torch over the probe, two things happen - first, it gets hot, so the 'battery' function works. And the flame of the torch has very little oxygen in it, so there is a big difference in oxygen between the front and rear of the sensor, so you see a voltage.

So what you're seeing is the normal operation of an O2 sensor. In fact, one reason O2 sensors fail is that the face gets gunked up with contaminants, and cleaning them off with a torch is one way to remove them.
The other thing O2 sensors do as they age is slow down. You won't see that difference with a torch. A good O2 sensor can shift in as little as a millisecond, and can actually detect differences between cylinders firing. But as they age, like the rest of us, they slow down. If they slow down enough, the control system gets whacked. The ECU puts richens the mixture, but the O2 sensor doesn't respond right away. So the ECU adds more fuel. Then more. Finally, the O2 sensor responds, so the ECU starts to cut back. But the O2 sensor is still going towards rich, although the mixture is now lean. The ECU and the sensor get out of sync, and the mixture is all over the place.


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