Hello my engine light came on I ha in plugged in and code p0420 came up. I have a 2005 jeep grand Cherokee lardo trail rated edition. Now my question is what causes this. My jeep is running fine I didn't notice any problems. While I was driving it the light came on and I checked the oil and had almost no oil. The oil light didn't come on to let me know and I just had an oil change last month. There is no oil leaking onto the ground so not sure what happened. It says it could be the oxygen sensor, catalytic defective due to #2 3 or 4, engine misfire or running condition, large vacuum leak, engine oil leakage into exhaust-valve guide piston rings. Can anyone help me don't want to fix something when it we'll could of needn't something else. Thank you for your time!!