I just thought I would share this in case it helps anyone.

I began receiving the CEL code P0420 (catalyst system efficiency below threshold.) which started occasionally (once a month in damp conditions) and progressed to appearing every 3 days of driving (over 2 years) in any condition. I was simply clearing the code (via enginelink app) as I was told it ain't serious and is only emission related.

I troubleshooted all forum suggestions to no avail and it was all pointing to the exhaust catalytic converter going bad.

Well, I changed the plugs and as my car also developed uneven idle I've had my throttle body cleaned as part of my service and... The CEL code is now gone.

I suspect it was the plugs.

So when you get this code change your plugs first.

 

it maybe gone for a month or so.

 

And then what? New cats?