A few days ago my CEL came on. I disconnected the battery and it went off. I figured it was a fluke. It came back on today. The code I got from the dianostics test is p0420. I know this can be 02 sensors, cat, or a leak somewhere. I figured I would try to clean the first 02 sensor. When I took the heat shield off I saw a hairline crack in the exhaust manifold. Also the sensor wouldn't budge and with the day I've had I put the s**t back on and said f**k it. One of the bolts was broken off the heat shield which leads me to believe the 02 sensor has been replaced already. I have only had this car for 3 weeks but the people before me took good care of it. So now I am thinking the problem is the cat or that hairline crack...anyone have any advice before I take it to a shop?