Hi, since about 2weeks the engine control light from my Ford Ka BJ.2005, 1.3….now I had ordered a Bluetooth readout adapter that came today, immediately started and there I got the error code P0420: what iwas must have to do with the exhaust system(according to research)….now it can be the cat, Lambdasonde or iwas with the air supply….has someone an idea how I can test it myself or fix it….a new cat costs fast times 150-350€….that the car doesn’t even have more Value….what can I do? thank you! LG