The way I understand it is that the pre-cat sensors control the mixture. If the mixture went out of the closed loop range or anything was suspicious about their operation they should raise a different code.
The post cat sensors are behind the 1st catalytic convertor (of 2) in the system (in the exhaust elbow just under the manifold)but before the main cats. These sensors will behave the same way as the pre-cat sensors until the 1st stage cat starts cleaning the exhaust gases, when they will just read rich (since the sensors work by measuring excess oxygen in the exhaust and the cat uses up the oxygen breaking down nasties).
I suspect the ECU is looking for the 1st stage cats to start working after a warmup period (maybe using a richer mixture, retarding ignition, delaying gear changes, etc. until they have woken up).
When mine has lit the emissions light it has been shortly after the engine has warmed up, although once that coincided with filling from empty from a petrol station I don't usually use.
I wonder if the 1st stage cats fail to work after the specified time sometimes, or allow some dirty gases through during high load, and such events are counted up and cause the light to come on if it happens too frequently?
I have seen the two codes in my ECU without the light having come on too, so it could be that the problem gets logged at one level and lights the light if it persists.
In any case, once the main second stage cats start working the exhaust emissions are fine but there is no monitoring of the second stage cat efficiency.
Kevin
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Topic: Cats on the way out? (Read 3784 times)