I'm suprised that it could fail the emissions test because of a problem with the pre-cats (in the manifold), because some LOC members have deliberately removed those pre-cats, keeping the main cat, and passed the MoT still.
If you have a look at the link in my earlier response, the diagram shows the exhaust manifold towards the top left. This is basically the start of the exhaust system, there's a pipe leading from each cylinder (6 on the IS200), and each of the "banks" of cylinders (two banks of cylinders with three cylinders each) feed into their own pre-cat (i.e. small catalytic converted). This is all the assembly labelled "Exhaust Mainfold (with TWC)" in that diagram.
The manifold then connects onto the main cat, which is shown at the bottom right of that diagram. This is a much bigger cat, which I would presume does the main work in cleaning up the emissions.
Assuming all the diagnosis is correct, then I wonder if the main cat is pretty marginal too ?
Anyone know more about this ?