Comment by Panoramix
12 hours ago
Yes but how do you do that? that magical third electrode sounds harder to make than the original problem.
Edit: I think I get it now, it's a chemical reaction. By applying a voltage with some polarity to the 3rd electrode you can run the reaction in reverse. Still very hard to achieve because you have to make sure the reactions happen at the same rate with the same efficiency, which is far from trivial. This must be a very high end sensor for all this effort to make sense.
An oxygen molecule does some chemical reaction on the sensor electrode that releases an electron, maybe it's made of iron and turns into rust. If you supply the same current to another electrode to do the opposite reaction, maybe one made of rust that turns into iron, it balances.
The sensors must be consumable with a certain lifetime.