P0420

CIRCUIT DESCRIPTION
In order to maintain a reasonably low emission level of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and CO present in the exhaust gas. This reaction converts the gases into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx , converting the NOx into nitrogen. The engine control module (ECM) monitors this process using the post catalyst heated oxygen sensor (HO2S) signal. The post-catalyst HO2S located in the exhaust stream after the TWC, produces an output signal that indicates the oxygen storage capacity of the catalyst. The oxygen storage capacity determines the ability of the catalyst to convert the exhaust emissions effectively. If the catalyst is functioning correctly, the post-catalyst HO2S signal will be far less active than the signal produced by the pre-catalyst HO2S. If the ECM detects an excessive post-catalyst signal activity outside of an acceptable range for an extended period of time, the ECM will set this diagnostic trouble code (DTC). This indicates that the TWC oxygen storage capacity is below a threshold considered acceptable.

CONDITIONS FOR RUNNING THE DTC
- DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0101, P0102, P0103, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0150, P0151, P0152, P0153, P0154, P0155, P0156, P0157, P0158, P0160, P0161, P0221, P0222, P0223, P0300, P0301-P0306, P0313, P0440, P0442, P0443, P0444, P0445, P0446, P0447, P0448, P0449, P0496, P0638, P2096, P2097, P2098, P2099, P2100, P2101, P2105, P2107, P2119, P2122, P2123, P2127, P2128, P2135, P2138, P2176 are not set.
- The engine is operating in Closed Loop.
- The calculated TWC temperature is between 520°C (968°F) and 790°C (1454°F).
- The engine load is between 25-30 percent.
- The engine speed is between 1,200-3,000 RPM.

CONDITIONS FOR SETTING THE DTC
The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the 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

Steps 1-2:

Steps 3-5: