I just got a cat back exhaust done on my Bf SII Xr6 na and noticed a loss in power compared to the stock one when i put my foot down in 2nd and 3rd it shudders and takes a few seconds to respond.
Any tips on what i could do to stop this would be appreciated.

Check the 2nd O2 sensor?

what should it look like?

Like the 1st oxygen sensor hopefully.... It's just another one mounted behind the catalytic converter. Can cause tuning issues when not functioning. Not an issue with ba's.

I ment how could i check that its working properly should of been more specific

best thing I ever did for my bf was to buy a cheap scantool off ebay. It will tell you whats wrong & where.

You can check the sensor with a multimeter but you have to know which wires you're checking and what to look for. You want to check the signal wire for 0-1V iirc. 0 being rich 10 being lean or vice versa, can't remember but it will be fairly obvious.

actually if i remember rightly the boys that throw on after market cats get check engine lights (mil?) from throwing out the second sensor so if it's down it should throw a light.