engine management light

hi, I recently had a new middle and back box done on the exhaust of my facelifted 1.6 45.Now pretty much (within 50 miles) I had the engine management light come on, I used my scanner and it said PO136 , which it says is a O2 sensor malfunction (Bank 1 sensor 2) , I've replaced the pre cat sensor with a genuine rover one, but again within 50 miles or so , the light was back on, and again with code PO136. Does anybody have any ideas ???? I was thinking that it maybe the catalytic converter itself that's at fault, I can see no obvious loose wires anywhere, so I'm stumped. Thanks in advance for any advice.

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