Went to AutoZone, got my code read, came out with P0420 Catalyst Efficiency Low-Bank 1 Explination: The oxygen sensors monitor the Catalytic Converters ability to store oxygen Probable Cause: 1. Catalytic converter defective (Failure possibly due to #2, 3, or 4) 2. Engine misfire or running condition 3. Large vacuum leak 4. Engine oil leaking into exhaust valve guide (reciept's ripped here) ..rings (They printed it off for me, and the reciepts ripped after guide, but theres like 2 words before "rings") I dont get it, are my cat's bad? Or do I have a large vacuum leak? Or is my oil leaking into the exahust...? How can I narrow this down and fix it? 2000 LS V8, with 174,000 miles.