i take there is no lambda in that section(3rd)
but is there 4 in maniverter 2 before and 2 after cats?
Bang on, all 4 lambdas are in the maniverter, one before each cat and one after.
A couple more questions:
Can the lambda readings be ok even if one of the first cats is knacked?
Is the 3rd cat needed to pass the MOT?
How can I isolate the problem without changing expensive components?
(Well, three questions
1) If the cats aren't working, the secondary lambdas should pick up on it and throw up a fault light. The lambdas could be faulty in such a way that no fault code is thrown up for the cats, but also that no fault code is thrown up for the lambdas. Also, the lambdas could be working fine, but not picking up that your cats aren't working due to problems with the wiring loom for the lambdas.
2) Probably - your engine would have to be running absolutely spot on to get through the emissions test with only the manifold cars. I do know of a GTA that has had the 2 main cats removed and passed an MOT emissions test with only the manifold cats fitted, so it is in the realms of possibility.
Legally, you could remove the cat converter as your car still has two others fitted, so with the 3rd one removed you'd still be complying with the legislation that states you must have a cat converter fitted. But it would all come down to the MOT emissions test. What area of the emissions did it fail on? Lambda, CO or HC?
3) This is the key really - swapping known-good used components one at a time in a bid to eliminate the problem is one way of doing it. JTS engine running problems are notoriously hard to diagnose because there is so many potential causes of the problems, from the ECU, camshafts, coked up inlets, internal engine wear, maniverter issues, lambda sensors, wiring loom.. It is hard to know where to start if you aren't experienced with the JTS (and I'm not - my experience comes mostly from the parts people have bought to fix the issues they've been having with these engines)..