Fault code P0420 is for catalyst below efficiency threshhold. This normally means a faulty catalytic convertor.
The catalytic convertor is covered by the Federal Emissions Warranty for 8yrs or 80,000 miles, so if you are under the mileage the convertor will be replaced under warranty at the dealership.
there is a TSB(technical service bulletin) on this:
26 07 05
Sept. 5, 2007
MIL is ON and/or Catalytic Converter is Rattling
MIL is ON, DTC P0420 is stored and/or catalytic converter rattling.
The expansion mat in the original part can erode prematurely, causing the converter to rattle and/or MIL ON.
A new part was designed to fix this concern.
Repair identified after production was completed.
Prior to replacing a catalytic converter under the Catalytic Converter Limited Extended Warranty, perform the following procedure:
Oil level MUST be correct.
Adjusting oil level is NOT covered by Warranty.
Is Catalytic Converter Rattling?
If catalytic converter is NOT rattling:
If catalytic converter is rattling:
If signs of external damage to catalytic converter exist:
The cost of replacement catalytic converter is NOT covered by warranty.
Interrogating DTC Memory
Address 01 fault memory including ECM information.
"OBD" freeze-frame modes 2 and 6.
For OBD, see ElsaWeb, Repair Manual - Fuel Injection & Ignition 2.0 Liter 4Cyl. 2V Generic Scan Tool.
Steps for printing "OBD" from the touch screen:
Address 01 Measuring Value Block (MVB) 46.
The DTC codes present will need to be erased before the test will run in MVB 46.
Is DTC P0420 (Catalyst System Efficiency Below Threshold, Bank 1) Stored?
If catalytic converter is NOT rattling and DTC P0420 and any DTC(s) from table are stored:
If catalytic converter is rattling and any DTC(s) from table stored:
If catalytic converter is NOT rattling and P0420 is the only DTC stored:
If catalytic converter is rattling and P0420 is the only DTC stored.
No DTCs are stored:
DTCs in table are stored:
The reason for DTC(s) from Table MUST be corrected (possibly at customer expense) prior to potential catalytic converter replacement.
With other DTC(s) corrected:
If complete readiness code cannot be set due to the catalyst diagnosis monitor in display group 46 not resetting:
If complete readiness code can be fully set:
If vehicle was returned to the Customer (without converter replacement) and Customer returns with DTC P0420 stored again:
Replace catalytic converter
Catalytic Converter Limited Extended Warranty:
Required Parts and Tools
When replacing any catalyst, your VIN may be required to ensure the correct part is ordered (check with your Parts Dept.).
If catalytic converter is on parts submission:
Claims for Catalytic Converters (which are on submission) that are shipped to the Warranty Test Center without print-outs (including freeze frame data) may be denied.
All part and service references provided in this Technical Bulletin are subject to change and/or removal.
Always check with your Parts Dept. and Repair Manuals for the latest information