i have a 97 v6 auto with 112,xxx miles. My check engine light keeps coming on... and has been for the past 10000 miles... i went to autozone since i dont have a scanner... and it was code p0420

definition- catalyst efficiency low-bank

explanation- o2 sensors monitor the cat ability to store oxygen

probable cause
1. cat defective
2. engine misfire or running condition.
3. large vacuum leak
4. engine oil leakage into exhaust-valve guide seals, piston rings.

i have checked the plugs recently and they look good, and 2 of the 3 coils has been replaced within the last 20,000 miles. and, my fan and where it blows seems to be working fine still, so i dont think it is a vacuum leak or misfire unless its the wires...

what do you think i need to do?