2004 STi (Type UK): Code P0420 came on during road trip

[Skipper555]

Hi guys, so check engine light came on during my road trip. Code is: P0420.

Took it to a local mechanic and he welded the flex pipe thinking it was the solution, but after driving about 200 miles, it came back on today.

Current Mileage: 111437 (miles)
Symptoms: Car drives smoothly, but the lack of boost is noticeable when I get near 4k rpm (the typical punch/kick is not there).

Anyone came across this before? Is this purely linked to the catalytic converter or could it also be the turbo?

Will take it to the mechanic again in 2 days.

Thanks!

 

[BazGF8]

Wouldn’t effect boost levels a whole lot but after a quick google here is the result , you need someone with a bit more experience to look at live readings from the ecu
is the cat fitted to the exhaust?

 

[scoobysub]

Could be a few things really lamba sensor . Cat is blocked
Unlikely the leak back that far caused the issue . Is the rear lamba sensor Seoul connected