I understand the theory behind a wideband, but I don't understand the theory behind how an exhaust leak will cause a rich condition. Do you think that only O2 will leak out of the system?

If you think that it is just unpredictable what will happen to the O2 measurement, then I would contend that if that were true then a case must be made for multi-point measurement of the exhaust gas across the cross-section of the pipe for an accurate measurement.

Believe it or not, in another thread someone made the claim that an exhaust leak would cause an O2 sensor to read leaner than actual.

 

It's not the fact that you're making it run too rich or too lean even. It's the fact that it's inaccurate. When tuning your car, you obviously want things as accurate as possible to let you know where your limits are correct? Well if exhaust is escaping the system then yourr readings aren't exactly going to be 100% spot on.

 

I am asking what the theory is. You have just repeated the same thing.

Why does an exhaust leak cause the error?

 

The O2 measures oxygen in the exhaust. If you have a leak, more oxygen can leak into the exhaust and throw off the reading. It would make you show leaner than you really are.

 

Let me get this straight. You are saying that more oxygen causes a leaner reading. However, I have a question. Is that caused because if more oxygen is left over after combustion, the 02 sensor will "know" that more fuel is needed to combat the "extra" oxygen? In other words, the 02 reading is saying "You have too much oxygen, add fuel!"?

 

How does O2 leak INTO the exhaust? That won't happen. Your exhaust will only leak out, no O2 will leak in. Think about it.

Cetchup...

What you have said is correct. A wideband O2 is measuring the excess O2 (the extra O2 that was not combusted), so it is telling you there is too much O2 --> add fuel.