Ford street Ka engine light.

Hi everyone, I recently made a post here about a ford street Ka with a busted chassis but now have looked at the chassis myself and don't see anything and explored it to find out that it was the person that was responsible for collection of the vehicle that mixed up the words of the mechanic and then told me the wrong thing. Turns out it was just the engine mountings after I looked at it. I am now looking onwards to a engine light issue, I have placed an obd on it and got the reading that the O2 sensor was busted so I got it replaced and still getting the warning even after turning it off it just comes back. Interestingly I think it's only when it goes through water that the light comes back so I believe there is something exposed letting water in and so far I see a whole in the plastics in-between the shock absorber and the wheel arch that may be letting water through. The only thing I see that is there is fuses and none of them are blown so it's puzzling me as to what the issue is! Anyone have experience with this issue? By the way it's convertible model