E46 error code U0420 and a leak which looks like its around oil tank area(check engine light) what to do?

[dimi]

Recently bought bmw 318i 2004 convertable. The check engine light came up and after that i used an OBDII which showed one code "U0420". I opened up the hood and looks as if there is a leak just under the oil area. However the code is regarding invalid power stearing data. How can i be sure its an oil leak or power stearing fluid leak? Any help appreciated!