3.2 GT exhaust cat error

[Desmosedici]

Hi everyone
I have a problem with the cats bringing on the Engine Fault warning and light.
The Alfa dealer sent me to an aftermarket exhaust manufacturer, to try and save me some dollars.
The diagnostic tool isolated the 2 mid cats as faulty, but the exhaust was found to have debris from further upstream.
In short, how many cats are on this system (2006) and how many sensors need to satisfy the computer?
Has anyone had exhaust work done to this degree, and able to shed any light on what I may need to do here?
Thanks
Paul

[Citroënbender]

G'day, there are four cats and four sensors, the first two cats are sometimes known as pre-cats and the sensors in them operate to regulate mixture - not to check for catalytic process efficiency.  It's the trailing sensors that check overall effectiveness of the catalytic conversion.

Although not on a V6, I have had problems with a poor harness connection affecting the lambda sensors, the connector just needed its pins tightening up. Whenever you hit a decent pothole, it would come up with the dreaded "Motor Control System Failure" and a cat fault logged.

[Desmosedici]

Thank you very much for your reply, sir.
Is it possible to replace the precats with an after-market product, or eliminate them altogether?
The Engine Monitor System Failure message needs to be satisfied and eliminated.
Thanks, I really appreciate your help.

[Citroënbender]

I'd wager your precats are metallic substrate, you can't bash these out like a ceramic honeycomb cat. Plus, when they block up, they block up pretty well.

Speaking generally, precats can be filleted by a skilled TIG welder, ideally they have a stainless tube welded in that is the same as the inlet/outlet diameter, and then the exterior stitched up neatly. Note that legality generally revolves firstly around removal or modification of emissions control equipment, and much more secondarily around emissions.  If this (tube insertion) is not done there is a characteristic "decat" sound which can be heard especially at low revs or with a cold motor. If you choose to decat, please remember the exhaust fume hazards (we have all become complacent in the last 20 years!) and keep the work under your hat.

Eliminating the front cats may place more stress on the rear converters as a rich mixture will not have started its catalysis process up the front. You might satisfy the ECU with aftermarket high flow cats to replace the rear pair, but I wouldn't want to be held to it. It may also be that if you gutted the rear cats and modified the precats as described, your ECU will just carry on regardless, not troubling you with a fault light.

If you're inclined to tinker, grabbing a full spare set of cats and lambda sensors off a "known good" motor, may be one way to pinpoint the faults.

[Desmosedici]

Great info, thank you!
When the warning annunciated 2 weeks ago I took it to the usual Alfa center and they diagnosed it as the rear cats. The service guy thought that the cats could be more easily and less expensively replaced by a custom exhaust firm.
The custom exhaust fellow was a nice bloke, and ordered 2 rear non OEM Euro-spec cats.
When he opened the mid-pipes to replace the cats, he found debris (looked like normal cat metal material, and some other very heavy metal he thought was a metal bi-product)
He closed it all back up and gave me the news that the job was a lot more complicated than a simple replacement of the mids. He couldn't find the pre-cats, given the time then available.
He is apparently searching for cat ratings and resistance specs for the ECU, but he's not an Alfa guy, so I don't like my chances on the aftermarket solution working out.
I'd like eliminate the pre-cats and replace the mids with a spec that will satisfy the computer (if the ECU is not effected by doing that) 
Any further suggestions of how I should proceed would be greatly appreciated.
Again, thank you very much for the excellent info, I will pass it on and see what the exhaust fellow thinks he can do.
Cheers
P