I believe your are referring to P0420 and not as indicated. Some after market catalytic converters do not match up and would cause the coede to return even if it is new.
Here are the diagnostic procedures and what to check first.
DTC P0420: CATALYTIC SYSTEM EFFICIENCY BELOW THRESHOLD
If DTCs P0137, P0138, and/or P0141 are indicated, trouble shoot DTC P0420 last.
DTC P0420 is set and MIL illuminated when malfunction has been detected during 3 consecutive trips.
1. Perform PCM reset procedure. Start engine and run at 3000 RPM with no loads and transmission in Park or Neutral until radiator fan comes on. Connect SCS service connector (07PAZ-0010100). Test drive
vehicle at 40-55 MPH for about 2 minutes. Decelerate for at least 3 seconds with throttle
2. Reduce vehicle speed to 35 MPH, and hold it there until readiness code comes on. If DTC P0420 is not indicated, problem is intermittent. System is okay at this time. If DTC P0420 is indicated, inspect Three Way Catalytic Converter (TWC) for plugging, melting, cracking, or exhaust system leaks. Repair or replace TWC as necessary.
Sunday, February 14th, 2010 AT 6:08 AM