Celica Hobby - Check Engine Light P0420

here's the text for the P0420 code...

P0420
After the engine and the catalyst are warmed up, and while the
vehicle is driven within the set vehicle and engine speed range,
the waveforms of the oxygen sensors (bank 1 sensor 1 and
bank 1 sensor 2) have the same amplitude
(2 trip detection logic)
Trouble area..
Gas leakage on exhaust system
Open or short in heated oxygen sensor circuit
Heated oxygen sensor
Three-way catalytic converter

INSPECTION PROCEDURE
HINT:
Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool. Because freeze frame records
the engine conditions when the malfunction is detected, when troubleshooting it is useful for determining
whether the vehicle was running or stopped, the engine warmed up or not, the air-fuel ratio lean or rich, etc.
at the time of the malfunction.
1 Are there any other codes (besides DTC P0420) being output?
YES Go to relevant DTC chart.
NO
2 Check gas leakade on exhaust system.
NG Repair or replace.
OK
3 Check oxygen sensor (bank 1 sensor 1) (See page DI-49 ).
NG Repair or replace.
OK
4 Check oxygen sensor (bank 1 sensor 2) (See page DI-58 ).
NG Repair or replace.
OK
Replace three-way catalytic converter.

basically it could be either your 02 sensor OR your cat.