This info is from the 2001 Service CD but I believe it is still applicable:
HF1 DTCs P0420 OR P0430: CHECK FOR MISFIRE DETECTION MONITOR DTCS
NOTE 1: Be sure customer has not:
(1) Refueled vehicle with leaded gasoline.
(2) Noticed high vehicle oil consumption.
NOTE 2: If entering this Pinpoint Test for symptoms only, immediately GO to «HF5».
NOTE 3: Internal deterioration of a catalytic converter is usually caused by abnormal engine operation upstream of the catalyst. Events that can produce higher than normal temperatures in the catalyst are particularly suspect. For example, misfiring can cause higher than normal catalyst operating temperatures.
l Retrieve and record all Continuous Memory DTCs (MIL and non-MIL).
l Were any of the following Misfire Detection Monitor DTCs recorded: P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0309 and P0310?
Yes
GO to Section 4, Powertrain«Diagnostic Trouble Code (DTC) Charts», to address the Misfire Detection Monitor DTCs.
No
GO to «HF2».