I have a 2000 Jaguar S Type with a P0420 code that is showing up. I have a nice odb tool that is showing bank 1 sensor 1 as fluctuating and bank 1 sensor 2 as fluctuating but on bank 2 sensor 1 is fluctuating and bank 2 sensor 2 is pretty steady. I personally have replace the O2 Bank 1 sensor 2 behind the cat and it appears that the bank 1 sensor 1 has already been replaced. From what I understand the previous owner replaced the coils at some point but not sure when or if it was all of them. Any thoughts on what could be wrong or what to check? Also I have noticed that it does occasionally idle a little rough but its not really bad just a little shake here or there.

P.S. Car only has 39,000 miles on it and it takes it a few days before the Engine light comes back on.

 

Just an FYI this is the V6 3.0

 

I would think a vacuum leak first (check fuel trims at idle and 2,500 RPM), then I would look at the coils - bank 1, especially with your description of a little shake here and there - those "misfires" are dumping unspent fuel through the manifolds and onto the cat - causing damage.

P0420 fault code, catalyst efficiency below threshold bank 1 (I believe passenger side)

If you have no vacuum leaks and your coils check out (no oil on coil boots) - look at the O2 sensor between the cat and the exhaust manifold.

If all the above checks out - then you have a bad cat needing replacement.