P0420

Description
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic (TWC) converter is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HO and the CO that is present in the exhaust gases. The oxidation process converts the HO and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx to nitrogen. The powertrain control module has the ability to monitor this process using the HO2S 1 and the HO2S 2. The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gases entering the TWC converter. The HO2S 2 produces an output signal that indicates the oxygen storage capacity of the catalyst in the TWC converter. The oxygen storage capacity of the catalyst is an indicator of the converter's ability to convert exhaust gases efficiently. The HO2S 1 output signal will be far more active than the HO2S 2 output signal when the converter catalyst is operating efficiently. A DTC P0420 will set if the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently.

Conditions for Setting the DTC
Calculated deviation rate:
^ Engine coolant temperature between 70°C (158°F) and 110°C (230°F).
^ Intake air temperature between -10°C (14°F) and 70°C (158°F).
^ Barometric pressure greater than 75 kPa.
^ Fuel tank level greater than 15%.
^ Engine speed is between:
- A/T 1,500 and 3,500 RPM.
- M/T 2,750 and 3,750 RPM.
^ In closed loop mode 139 seconds at 3,000 RPM from engine start.
^ Above conditions present for 96 seconds.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.
^ The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC
^ The MIL turns OFF after three consecutively passing trips without a fault present.
^ A History DTC clears after 40 consecutive warm-up cycles without a fault.
^ Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC.

Diagnostic Aids
Check for any of the following conditions:
^ Exhaust gas leaks.
^ HO2S 1 and HO2S 2 malfunctions. Repair all HO2S DTCs before diagnosing a DTC P0420.
^ A damaged wiring harness. Inspect the HO2S 1 and HO2S 2 wiring harnesses for damage and repair as necessary.
A TWC converter failure may be caused by any of the following conditions:
^ Engine misfire
^ Retarded ignition timing
^ Inadequate secondary ignition voltage
^ Excessive engine oil or engine coolant consumption
^ Rich engine fueling
If a DTC P0420 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
1. The Powertrain (OBD) System Check prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. Perform a visual/physical inspection of the catalytic converter.
5. In order to avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.
6. A relatively inactive HO2S sensor 2 indicates that the converter is minimizing the amount of oxygen passing through the converter.