Intermittent P0420

I started seeing this error in my code scanner about a month ago. I tried some Seafoam, and the check engine light went away.....then came back a week later. Another can of seafoam - check engine light stayed on.

Today I came back from a 4 day holiday, started it up and the engine light was OFF. Now what! Am I dealing with bad oxygen sensors (no codes for them are appearing), stuck fuel injectors (short term and long term readings look pretty good) or a weird catalytic converter? It's a 2003 4.7L with about 98k miles. Suggestions, before I starting spending my hard-earned dough?

 

I would replace the Oxygen sensors just before and after the Cat(s). I would not imagine the catalytic converter is already 'worn out' with just 98k miles. Every time I have ever seen the trouble code come up it is resolved by either cleaning the cats or replacing them.

 

It could be as simple as an exhaust leak the the vicinity of one of the sensors and they're reading too much O2 in the stream. It might require two people, but with the engine idling cover the tailpipe with gloved hands and a towel or rag and listen for the little puffing sounds around connections....worth a try and is free.

 

Thanks! I will try the tailpipe option this weekend and report back.