http://www.omegaowners.com/forum/YaBB.pl?num=1223977030/0
The only purpose of these "post-cat" Lambda sensors is to determine if the pre-cats are working effectively and to light the emissions light if there is a problem. They are not involved in fuel trim at all - for a very good reason. The fuel mixture is measured by the front sensors by detecting the proportion of unused oxygen in the exhaust gases following the chemical reaction that occurs in the cylinder. Put a rich mixture into the system - one with excess fuel - and all the oxygen will be used up burning this fuel. Put too little fuel into the cylinder and the fuel will all have burnt before the oxygen is used up. Once the exhaust gases have passed through a catalytic converter, further oxygen is used up in the process of breaking down more completely the oxides of nitrogen and carbon monoxide, and this no longer is able to tell us about the mixture.
So, back to the cat efficiency errors. If the pre-cats are working, and the front lambda sensors are, through the fuel trim process, keeping the engine's mixture correct, there should be no or very little oxygen left in the exhaust after the cat. A chemically correct mixture means that there is exactly enough oxygen flowing into the engine to fully break down the fuel. Most of this breakdown happens in the engine itself, as the fuel is burnt, and the rest occurs in the pre-cat as some of the harmful products of part-burnt fuel are removed, leaving no oxygen at the post-cat lambda sensors. Due to the principle on which a lambda sensor works, a sensor seeing no excess oxygen will always read "rich".
So, the ECU can expect the pre-cat lambda sensor always to be "cycling" between rich and lean as the continuous process of adjuting the fuel trim occurs. For a while the post-cat lambda sensor will follow this cycling as the exhaust gas passes straight through the cat. As the cat reaches working temperature, however, the oxygen will start to be removed from the gases and the post-cat lambda sensor will start indicating "rich" for a greater proportion of the time until it is reading "rich" 100% of the time.
