P0420

Circuit Description
In order to maintain a reasonably low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the engine controls system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. This reaction converts them into harmless water vapor and Carbon Dioxide. The catalyst also reduces NOx, converting it to Nitrogen. The PCM monitors this process using Bank 1 HO2S #2. Bank 1 HO2S #2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the Bank 1 HO2S #2 signal will be far less active than that produced by Bank 1 HO2S #1. This indicates that the three-way catalytic converters oxygen storage capacity is below a threshold considered acceptable.
The PCM performs this diagnostic at idle when the conditions for running this DTC are met, the following occurs:
1. The PCM captures the current rear HO2S rich/lean status.
2. The Air Fuel ratio transitions from rich to lean or lean to rich depending on the capture rear HO2S rich/lean status.
3. The Air Fuel ratio transitions a second time opposite the first Air Fuel ratio transition.
4. The PCM captures the response time (the time the HO2S goes from below 300 mV to above 600 mV and from 600 mV to below 300 mV) of the front and rear Heated Oxygen sensors when the Air Fuel ratio transitions occurred.
5. The PCM measures the time it takes the rear HO2S voltage to cross a reference rich/lean threshold minus the time it takes the front HO2S voltage to cross the same rich/lean threshold. The time difference from the front and rear HO2S is the oxygen storage capacity of the catalyst. This DTC sets if the time exceeds a predetermined threshold.

Conditions for Running the DTC
^ No AIR, CKP, CMP, IAT, IC, MAF, MAP, ECT, TP, EVAP, Left Bank HO2S, Misfire, Left Bark Fuel Trim, Injector, VSS, Engine Protection, or Transmission DTCs are set.
^ The IAT is greater than -15°C (5°F), but less than 75°C (167°F).
^ The Baro is greater than 74 kPa.
^ The engine is operating.
^ The ECT is greater than 80°C (176°F), but less than 120°C (248°F).
^ The engine speed is greater than 850 RPM for a minimum of 17 seconds for manual transmissions and 27 seconds for automatic transmissions since the end of the last idle period.
^ The closed loop fuel control enabled.

Conditions for Setting the DTC
The PCM determines oxygen storage capability of the catalytic converter has degraded below a calibrated threshold.

Action Taken When the DTC Sets
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Freeze Frame and/or the Failure Records.

Conditions for Clearing the MIL/DTC
^ The PCM turns the MIL OFF after three consecutive ignition cycles that the diagnostic runs and does not fail.
^ A last test failed (current DTC) clears when the diagnostic runs and does not fail.
^ A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.
^ Use a scan tool in order to clear the MIL/DTC.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
4. Before replacing the Three Way Catalytic Converter, correct any conditions which may have damaged the catalyst.