2011 mini O2 sensor codes
 

I have a 2011 Mini with a header that codes 276A. (The catalytic converter conversion downstream O2 sensor.) I have tried 3 different after market cats to rid the car of the light coming on. Normally have to drive it 10-15 miles before the light comes on after clearing the code. I have tried the extensions on second O2 sensor it did not remove the code. Any ideas beside trying to find original manifold and cat?