This is actually a known problem with Toyota and they have issued a Technical Service Bulletin on this issue. It is covered by an 8yr or 80,000 mile warranty and does require replacement of the ecm, along with new software programmed into it. Because the software is proprietary, it can only be done by a Toyota dealership. Not even us independent shops have access to this software.
October 19, 2005
Title:M.I.L. "ON" DTC P0420
Models:'01 - '03 Sienna
Under certain driving conditions, some 2001 - 2003 model year Sienna vehicles may exhibit a M.I.L. "ON" with the following DTC:
The Engine Control Module (ECM) (SAE term: Powertrain Control Module/PCM) logic has been modified to improve this condition.