2004 Other Toyota Models catalyitic converter

DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)
CIRCUIT DESCRIPTION
The ECM uses the air-fuel ratio (A/F) sensor and the heated oxygen sensor 2 to detect deterioration in the front catalyst.
Fig. 92: Catalyst System Efficiency Theory
Courtesy of TOYOTA MOTOR SALES, U.S.A., INC.
In order to detect deterioration in the front catalyst, the ECM calculates Oxygen Storage Capacity (OSC) in the catalyst based on voltage output of the heated oxygen sensor 2 while performing the "active air-fuel ratio control" instead of the conventional detecting method which uses the locus ratio.
The OSC is an indication value of the catalyst oxygen storage capacity and is used for representing how much the catalyst can store oxygen. When the vehicle is driving with a warm engine, the "active air-fuel ratio control" is performed for approximately 15 to 20 seconds. When it is performed, the air-fuel ratio is forcibly regulated to go LEAN or RICH by the ECM, and if a RICH and LEAN cycle of the heated oxygen sensor 2 is long, the OSC will become greater, and greater OSC basically capability of the catalyst are mutually related, the ECM judges if the catalyst has deteriorated based on the calculated OSC value.

If your mechanic can ead live data from the O2 sensors, have him look at the downstrem sensors for too much switching, when this happens, the converter is no good!

It should not be dependent on that, but a long trip will use more info like long term fuel trim estimates to make the judgement. You may want to have them monitor the O2 sensors to see if the downstream one may need replacement. PCM's(computers) are only as smart as the sensor input!

OK Your welcome.

Toyota has a tech bulliten out on this now, replacing the PCM and converter is what they recommend. Here is part of the bulliten:
M.I.L. "ON" DTC P0420 TECHNICAL SERVICE BULLETIN Reference Number(s): EG026-04, Â Date of Issue: Â October 8, 2004 TOYOTA: 2003 - 2004 Matrix (1ZZ-FE) SECTION: ENGINE Superceded Bulletin(s): EG026-04, Â Date of Issue: Â July, 12 2004 Related Ref Number(s): EG026-04 ARTICLE BEGINNING INTRODUCTION NOTE: October 8, 2004: Part number for replacement ECM (PCM) in Fig. 3 has been updated; Reference to section in Technical Information System (TIS) has been updated in "ECM (PCM) Replacement Procedure ". The previous TSB should be discarded. Some 2003 - 2004 model year Matrix vehicles equipped with a 1ZZ-FE engine may exhibit a M.I.L. "ON" condition with Diagnostic Trouble Code P0420 (Catalyst System Efficiency Below Threshold) stored in the Engine Control Module (ECM) (SAE term: Powertrain Control Module/PCM). Follow the repair procedure to reflash the ECM (PCM) and replace the catalytic converter assembly. NOTE: The repair procedure in this TSB is the same as TSB No. EG017-04 "Excessive Sulfur Dioxide Odor." This repair should only be performed once. APPLICABLE VEHICLES 2003 - 2004 model year Matrix vehicles equipped with the 1ZZ-FE engine produced BEFORE the Production Change Effective VIN shown below. PRODUCTION CHANGE INFORMATION PRODUCTION CHANGE INFORMATION