Got a P0420 DTC code right after new Fuel Filter.

[flamaest]

My mechanic says I have a bad catalytic converter, but I am thinking something else triggered this. HELP?!?

After reviewing a detailed Subaru catalytic converter diagnosis, I noticed there was a lot of mention of Fuel Pressure as part of the catalytic converter failure diagnosis.

Question, If I just changed the Fuel Filter on my STOCK 2000 Subaru Legacy GT [135k] and I JUST got a P0420 RIGHT AFTER, what is the chance my fuel filter is doing something weird/bad with my Fuel Pressure? Should I try another fuel filter first or is that a waste of time?

Also, I am running Platinum plugs installed about 30k ago, I heard this could cause a P0420.

Thanks,
Fabian