SHELBY GT 350 Member
Joined: Jan 2011
Location: Central Massachusetts
yes the leak would need to be upstream of the O2 sensors, or possibly at the O2 sensor itself . . . . here is the description I found on line that you probably already found too:
the P0420 code stands for “Catalyst System Efficiency Below Threshold (Bank 1).” When you receive a P0420 code it means your catalytic converter is not operating at maximum efficiency. Usually this means your catalytic converter needs to be replaced or there is a problem with your O2 (oxygen) sensors.
I doubt it is dirty MAF sensor, but that is a very easy thing to check -- it is in the intake tube just downstream of the filter, get some MAF cleaner (says on the can), remove the sensor, spray to clean; be careful you don't damage it and install it back in the correct orientation
| 2010 GT coupe | Daily Driver with some Steeda and GT500 take-off stuff for the occasional track day