2006 Quest P0420 code

Thanks Jim. Will a detailed diagnostic test help in identifying the root cause?

Appreciate your help, Thanks.

Hi Jim, I bought the Quest in 2013 with 130K kms on the odometer. I never changed spark plugs, just kept changing oil (synthetic) regularly. Also, I noticed during the last few oil changes that my mechanic (despite my concerns) poured some extra oil (more than the specified quantity) into the engine. Could these factors have contributed to the damage to the cat? Thanks.

Actually, I forgot to mention that I have driven it up to 185K kms now, so I suspect the old spark plugs might have been a factor. Some months ago, I used to get a subdued burning smell after a drive. Couldn't figure it out at that time.

I have also occasionally added fuel system cleaners to my gas tank, are they good or bad? Could the "cleaning" process be dumping stuff into the cat? I think the course of action now would be to have a diagnostic test done, replace plugs, cat and run it with premium fuel for some time to see how it performs. Thanks for your help. Highly appreciated.