Hey Bartiks, I just looked the P0420 code up in my repair manual and this is what it said.
DETECTION ITEM: Catalyst System Efficiency Below Threshold (Bank 1)
TROUBLE AREA: Gas leakage on exhaust system
Open or short in heated oxygen sensor circuit
Heated oxygen sensor
Three-way catalytic converter
If re-setting your ECU doesn't help, then those are the cause of the problem. If you are still under warranty, I would just take your car in if I were you because if it detected a problem with the oxygen sensor or catalytic converter, it could cost you a lot to replace if it failes later when you have no warranty left.
btw, good call Steven, sounds like you were right.