If you have euro spec manifolds (4 bolt flange) replace the pipe containing both the primary and secondary cats with a custom made or off the shelf solution with sports cats in the secondary cat position. Relocate lambda sensors, job done.
If you have the US spec manifolds (2 bolt flange) with the primary cats in the headers things become more complicated. You have a few options though:
1) Replace the US spec manifolds with euro spec ones and then do as you would for the euro spec above. Very expensive but the euro manifolds flow better so at least a small performance gain to go with your peace of mind.
2) Cut open the bottom of the US spec manifolds, gut the cats and weld them up again. Not ideal for the flow of gas in the exhaust but does deal with the problem effectively. Cut off the secondary cats and replace them with sports cats. There are no off the shelf options available for this (unlike the euro spec one) so you will either need to cut the original pipe or have an new one custom made. Then relocate the lambda sensors to the secondary cats.
3) The most common way of removing some of the worry is to simply replace the secondary cats and squish pipes with decat pipes, leaving the primary cats in the headers untouched. Not as full proof as the first two options but better than nothing.
All three options will free up the engine and may also add a little extra power, not to mention a whole lot of noise is you decide to mod the rear boxes or add a straight through X pipe.
Yes, some cars have manifolds with cats in (US-spec), others don't. What year is your car, Dave?
Hi Mark - mine is a MY 2006 (55 Plate) 4200
Dave
8 cylinder liners, 8 piston sets with rings and gasket kits cost 2600 gbp acording to Eurospares.
Not cheap but for someone looking for a project it might be worth it.
In the end the engine will be as new regarding compression ratios.
I would be grateful if you could point me in the direction of the IS that ran the diagnostics. I suspect that I am now the proud and stupid owner of this car, following a mad moment on ebay recently. It is currently with a specialist who is struggling to identify the problem - and the £notes are mounting for his time spent. Any help gratefully received.