im getting the engine managment light on, says o2 sensor, shall i just replace it do you think?
Replacing it won't do anything.
The problem you have is that you've decatted the car.
The reason these run 2 o2 sensors is to monitor if the cat is working or not. It should see a different result at the second sensor after the car, but because you've got no cat it's seeing the same gas and thinking the cat isn't working, hence your light.
This needs to be mapped out.
Some people have used spacers and the like with varying success, but to do it properly and ensure the light doesn't come back you need the map altered to suit.
