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 3-way catalytic converter (TWC). 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 the catalyst to nitrogen. The powertrain control module (PCM) monitors this process using post-catalyst heated oxygen (HO2S) sensors. The post-catalyst HO2S, located in the exhaust stream past the TWC, produce 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 post-catalyst sensor activity will be far less active than that produced by the pre-catalyst sensors. This indicates that the TWC oxygen storage capacity is operating at a threshold considered acceptable.

When the conditions for running this DTC are met, the following occurs:
1. The PCM captures the current rear HO2S rich or lean status.
2. The air fuel ratio transitions from rich to lean or lean to rich depending on the capture rear HO2S rich or 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 HO2S when the air fuel ratio transitions occurred.
5. The PCM measures the time the PCM takes the rear HO2S voltage to cross a reference rich or lean threshold minus the time it takes the front HO2S voltage to cross the same rich or 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
^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0125, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151, P0152, P0153, P0154, P0155, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0200, P0300, P0335, P0336, P0351- P0358, P0341, P0342, P0343, P0404, P0405, P0410, P0418, P0443, P0446, P0449, P0500, P0502, P0503, P1120, P1125, P1133, P1134, P1153, P1154, P1220, P1221, P1404, P1415, or P1416 are not set.
^ The engine has been operating for more than 600 seconds.
^ The engine coolant temperature (ECT) is between 70-158°C (176-248°F).
^ The intake air temperature (IAT) is between -15 to +75°C (+5 to +167°F).
^ The barometric pressure (BARO) is more than 74 kPa.
^ The fuel system is operating in Closed Loop.

CONDITIONS FOR SETTING THE DTC
The PCM determines the oxygen storage capability of the catalytic converter has degraded below 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.
^ Clear the MIL and the DTC with a scan tool.

TEST DESCRIPTION

Steps 1-6:

The number below refers to the step number on the diagnostic table.
4. Before replacing the TWC, correct any conditions which may have damaged the catalyst.