This is what my book say about this code
more testing is needed
DTC P0420: 1/1 CATALYTIC CONVERTER EFFICIENCY
NOTE:For component location, see COMPONENT LOCATIONS. For connector identification, see CONNECTOR IDENTIFICATION. For circuit identification and wiring diagram, see WIRING DIAGRAMS article. After each repair procedure has been completed, reconnect all components. Perform POWERTRAIN VERIFICATION TEST VER-5 under VERIFICATION TESTS to ensure system is functioning properly.
Monitored & Set Conditions
Catalytic converter efficiency is monitored after engine warms up to 158 F (70 C), throttle has been open for 180 seconds at vehicle speed more than 18 MPH, with engine speed at 1200-1700 RPM and MAP vacuum 15-21 in. Hg. DTC will set in Powertrain Control Module (PCM) memory when PCM senses catalyst efficiency deteriorated to a predetermined value. Possible causes for DTC to set are: intermittent condition, exhaust leak, engine mechanical problem, defective catalytic converter or upstream Heated Oxygen Sensor (HO2S) is older than downstream HO2S.
NOTE:1/1 HO2S may also be referred to as an upstream oxygen sensor. On 2.4L, 1/1 HO2S is located on exhaust manifold flange. On 3.3L and 3.8L, 1/1 HO2S is located on right exhaust manifold flange (manifold closest to firewall).
NOTE:If any O2 sensor DTCs are set along with P0420, diagnose those DTCs before continuing with this test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS.
Turn ignition on. Using scan tool, read DTCs. If Good Trip counter is displayed for DTC P0420 and displayed count is "0", go to next step. If Good Trip counter is not displayed for DTC P0420 or displayed count is not "0", go to step 7.
Inspect catalytic converter for damage, discoloration caused by overheating and for converter coming apart internally. Replace catalytic converter as necessary. If catalytic converter is okay, go to next step.
Start engine and allow it to idle. Check for exhaust leaks between engine and 1/1 HO2S. Repair exhaust system as necessary. If exhaust system is okay, go to next step.
Check exhaust for excessive smoke caused by coolant entering combustion chambers or high oil consumption. If excessive smoke caused by coolant entering combustion chambers or high oil consumption does not exist, go to next step. If excessive smoke caused by coolant entering combustion chambers or high oil consumption exists, repair engine as necessary and replace catalytic converter.
NOTE:A new downstream HO2S used along with an aging upstream HO2S may cause DTC P0420 to set.
If downstream HO2S has been replaced without replacing upstream HO2S, replace upstream HO2S. If downstream HO2S has not been replaced recently or downstream HO2S has been replaced along with upstream HO2S, go to next step.
At this time, catalytic converter is assumed to be defective. Replace catalytic converter.
NOTE:A new downstream HO2S used along with an aging upstream HO2S may cause DTC P0420 to set. Review vehicle repair history and repair as necessary.
Conditions for DTC to set do not exist or fault is an intermittent problem. Start engine and allow it to idle. Ensure engine is at normal operating temperature. Using scan tool, monitor parameters related to DTC while wiggling connectors and wiring harness. If value changes while wiggling connectors and wiring harness, repair connectors and wiring harness where wiggling caused value to change. If value does not change, review Freeze Frame data using scan tool. Attempt to duplicate condition that caused DTC to set. Check for any related technical service bulletins that may apply. Visually inspect related connectors and wiring harness. Repair connectors and wiring harness as necessary. If no problems are indicated, test is complete.
Monday, August 3rd, 2009 AT 8:33 PM