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Any diagnostic thinking appreciated!

I have a 2006 E85 2.0 z4. Back in January, I drove to work after 2 weeks of not driving, to find the car vibrating heavily on the motorway and then I received the P0420 cat warning. This was repeatable only after 10 minutes of driving - so suspected that I needed a cat replacement. This car has 3: 2 inbuilt in the exhaust manifold and one downstream. I replaced the exhaust manifold with a salvaged one that had only done half the miles (60k) and had been tested. After a local garage reset the adapters, everything was back to normal... until recently; less than 1000 miles later - another warning, and also a bit later, a low oil warning, with some oil evident top of the exhaust manifold.

Leaky rocker cover / valve gasket? Oil getting into the exhaust and ruining the cat? Or unrelated....
Any ideas welcome!

Hi Rojo,

Not a 4-pot (I4) owner, so take this as a more general comment from my BMW I6 experience.

I don't know your particulars, and how you came to cat replacement. At replacement, was testing was done to confirm it was the cat ? I replace upstream and downstream O2 sensors at the 100K miles mark, even if functional, because the zirconia sensor material will age and degrade with temp cycles (I'm a retired lab guy). Did your new cat get new Bosch sensors at install? Did they test the flow through the secondary cat in the exhaust in case it has crumbled and is restricting exhaust ?

External oil leaks are a pain, but that's probably a separate issue. It would be internal, half-burnt fuel or oil loading up your cat that would make it fail. Share the most detail you can so board members can give best advice. Members with I4 experience will hopefully chime in to advise.

Re-post this in the e85 problems section, you’ll get a lot more response