P0420

CIRCUIT DESCRIPTION
In order to maintain low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), the engine controls system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction, which oxidizes the HC and CO that are present in the exhaust gas. This reaction converts these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx which converts into nitrogen. The Powertrain Control Module (PCM) monitors this process using Bank 1 of the HO2S 2, which is located in the exhaust stream past the 3-way catalytic converter. Bank 1 of the HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalyst's ability to effectively convert the exhaust emissions. If the catalyst is functioning correctly, the Bank 1 of the HO2S 2 signal will be far less active than the signal that is produced by Bank 1 of the HO2S 1. This indicates that the 3-way catalytic converter's oxygen storage capacity is below an acceptable threshold.

The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs:
1. The PCM captures the current rear HO2S rich-to-lean status.
2. The air-to-fuel ratio transitions from rich to lean or from lean to rich, depending on the capture rear HO2S rich-to-lean status.
3. The air-to-fuel ratio transitions a second time opposite the first air-to-fuel ratio transition.
4. The PCM captures the response time of the front and rear heated oxygen sensors when the air-to-fuel ratio transitions occur. The HO2S response time goes from less 300 mV to more than 600 mV, and from more than 600 mV to less than 300 mV.
5. The PCM measures the time it takes the rear HO2S voltage to cross a reference rich-to-lean threshold, minus the time it takes the front HO2S voltage to cross the same rich-to-lean threshold. The difference between the front HO2S time and the rear HO2S time is the oxygen storage capacity of the catalyst. This DTC sets if the time exceeds a predetermined threshold.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101-P0103, P0106-P0108, P0112, P0113, P0117, P0118, P0125, P0131-P0135, P0137, P0138, P0140, P0141, P0151-P0155, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0200, P0300, P0325, P0327, P0335, P0336, P0341-P0343, P0351- P0358, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1120, P1125, P1133, P1134, P1153, P1154, P1220, P1221, P1275, P1276, P1280, P1281, P1285, P1286, P1441, P1514, P1515, P1516, P1517, or P1518 are not set.
^ The Intake Air Temperature (IAT) is more than - 15°C (+5°F), but less than 75°C (167°F).
^ The Barometric Pressure (BARO) is more than 74 kPa (10.7 psi).
^ The engine is operating.
^ The Engine Coolant Temperature (ECT) is more than 70°C (158°F), but less than 120°C (248°F).
^ Since the end of the last idle period, the engine speed is more than 900 RPM for a minimum of:
- 20 seconds for manual transmissions
- 30 seconds for automatic transmissions
^ The closed loop fuel control is enabled.

CONDITIONS FOR SETTING THE DTC
The PCM determines if oxygen storage capability of the catalytic converter has degraded to less than a calibrated threshold.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the DTC.

TEST DESCRIPTION

Steps 1-6:

The numbers below refer to the step numbers on the diagnostic table.
4. Before you replace the 3-way catalytic converter, correct any conditions which may have damaged the catalyst.