Bad catalytic converter or o2 sensors?

[NTidd]

Hey everyone! First time posting here and am hoping for some advice. My 14 year old son bought a 2006 TJ. It's a manual 2.4L and runs great but we are slowly repairing varies things along the way before he gets his drivers license.

The error codes we're receiving are:
P0420
P0456
P0137
P0138

Is it best to assume that the catalytic converter needs to be replaced or should we start with the O2 sensors first? You can easily smell fumes when starting the vehicle.

Any and all advice would be highly appreciated.

 

[Fishtaco]

I would start with the 02,s my 4.0 threw pre cat codes and I cleared them and tapped the 02,s a bit with a screwdriver and they have not come back for 4 months.
If its not sluggish under heavy load I would just change the 02,s and check your gas cap is fitted properly first.