P0420

CIRCUIT DESCRIPTION
The powertrain control module (PCM) tests the ability of the catalytic converter to lower the exhaust emissions. The PCM determines the performance of the catalytic converter by calculating the oxygen storage capacity. The PCM calculates this data by comparing the oxygen content of the exhaust entering the catalyst, read from the fuel control heated oxygen sensors (HO2S), to the exhaust leaving the catalyst, read from the catalyst monitor-HO2S sensor.

At the appropriate time, the PCM transitions the air fuel mixture from rich to lean or from lean to rich, depending on the current rich or lean state of the HO2S bank 1 sensor 2. The PCM then initiates a second, opposite transition. The PCM compares the time for the fuel control sensors to cross a calibrated threshold to the time for HO2S bank 1 sensor 2 to cross the same threshold. The longer it takes the HO2S bank 1 sensor 2 to cross the threshold, the higher the oxygen storage capacity. A HO2S bank 1 sensor 2 that crosses the threshold close to the same time as the fuel control HO2S sensors indicates a catalyst with a degraded oxygen storage capacity, and diagnostic trouble code (DTC) P0420 will set.

CONDITIONS FOR RUNNING THE DTC
- DTCs P0030, P0036, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0150, P0151, P0152, P0153, P0154, P0155, P0171, P0172, P0174, P0175, P0201-P0208, P0300, P0335, P0336, P0340, P0341, P0351-P0358, P0385, P0386, P0404, P0405, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P0452, P0453, P0502, P0503, P0506, P0507, P1031, P1032, P1133, P1134, P1153, P1154, P1336, P1359, P1360, P1372, P1415, P1416, P1441 are not set.
- The engine speed since end of the last idle period is more than 800 RPM for 46 seconds.
- The idle period is less than 60 seconds.
- The actual engine speed is within 100 RPM of the desired engine speed, for non-California vehicles.
- The actual engine speed is within 125 RPM of the desired engine speed, for California vehicles.
- The engine run time is more than 7.5 minutes.
- The predicted catalyst temperature is between 420-650°C (788-1,202°F).
- The barometric pressure (BARO) is more than 75 kPa.
- The engine coolant temperature (ECT) is between 71-120°C (160-248°F).
- The intake air temperature (IAT) is more than 6°C (21°F) and less than 200°C (392°F).
- The engine is in Closed Loop fuel control.
- Less than 6 tests have been attempted during this ignition cycle.
- One test has been attempted during the idle period.

CONDITIONS FOR SETTING THE DTC
The catalytic converter efficiency is too low.

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.

DIAGNOSTIC AIDS
Fuel with a high sulfur content can cause a marginal converter to look bad. Check the fuel quality.

TEST DESCRIPTION

Steps 1-4:

Steps 4-7:

The number below refers to the step number on the diagnostic table.
2. This step checks for conditions that can cause the catalytic converter efficiency to appear low. Inspect and repair any conditions before proceeding.