we bought the car from a person. the check engine light wasn't on then, try to get it to pass marder, it was a fuse the DMV marder place told me, we found the blown fuse put it in, went back & now the oxygen senitor was the problem. we put both of them in, but the light was still on. we drove the car around to set the new parts, never went out. we undone the battery , the light went out for a while, drove it around, the light came back on. took it back to marder now we have this catalyst system efficieny below threshold (bank 1) code p42. we have check the catalyst, no sign of any problems. we just want tags for the car! we knew the air condition did work but was going to get to it later. help!!!!!