P0420

CIRCUIT DESCRIPTION
A 3-way catalytic converter (TWC) is used in order to maintain low emissions of the following compounds:
^ Hydrocarbons (HC)
^ Carbon Monoxide (CO)
^ Oxides of Nitrogen (NOx)

The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that is present in the exhaust gas. This reaction converts the compounds into harmless water vapor and into carbon dioxide. The catalyst will also convert the NOx to nitrogen. The Powertrain Control Module (PCM) monitors this process via the signal from the post-catalyst Heated Oxygen Sensor (HO2S 2). The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. Oxygen storage capacity is one method of determining the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S 2 signal will be less active than the pre-catalyst HO2S 1 signal. If the PCM detects excess HO2S 2 signal activity for an extended period of time, the PCM will set this DTC. This indicates that the oxygen storage capacity of the TWC is below a threshold that is considered acceptable.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0030, P0036, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300 P0325, P0327, P0335, P0340, P0401, P0404, P0410, P0440, P0443, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351-P1366, or P1441 are not set.
^ The engine has been running for more than 9 minutes.
^ The IAT is between -18°C and +80°C (0°F - 176°F).
^ The ECT is between 75°C and 120°C (167°F - 248°F).
^ The BARO is more than 75 kPa.
^ The engine is operating in Closed Loop fuel mode.
^ The engine speed is more than 1000 RPM for more than 44 seconds.
^ The ignition voltage is more than 9 volts.
^ Warm up the catalyst:
- Fully open the hood.
- The transmission is in park (automatic) or neutral (manual).
- Set the parking brake.
- The engine speed is more than 1,500 RPM for 1 minute.
- Return to idle.
^ Test the catalyst:
- The transmission is in drive (automatic) or neutral (manual).
- Within 5-20 seconds, HO2S 1 should toggle rich above 600 mv for up to 7 seconds, then it may toggle lean below 300 mv for up to 7 seconds.
- Using the scan tool, check and see it DTC P0420 has passed or failed this ignition cycle.

CONDITIONS FOR SETTING THE DTC
The PCM determines that the catalyst oxygen storage capacity is below a threshold that is considered acceptable.

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 Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM turns the MIL OFF after 3 consecutive drive trips during which the diagnostic runs and passes.
^ A last test failed, or the current DTC, clears when the diagnostic runs and passes.
^ A History DTC clears after 40 consecutive warm-up cycles, if no other emission related diagnostic failures are reported.
^ Use a scan tool in order to clear the MIL diagnostic trouble code.
^ Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module (PCM) Description, Clearing Diagnostic Trouble Codes.

DIAGNOSTIC AIDS

NOTE: Refer to Connector Test Adapter kit J 35616-A Notice in Service Precautions

^ The PCM will not enable the catalyst test until the following conditions are met:
- The engine speed is plus or minus 100 RPM from the desired idle.
- The Throttle Position (TP) is 0 percent.
- The short term integrator is between -20 and +20 percent.
^ The catalyst test will abort if the vehicle falls outside the conditions listed below while the test is running:
- The engine speed is plus or minus 125 RPM from the desired idle.
- The Throttle Position (TP) is 0 percent.
- The short term integrator is between -20 and +20 percent.
^ The catalyst test may abort due because of a change in the engine load. If a load change occurs during the test, repeat the test.
^ More than 6 tests may have to be attempted to get 6 completed tests. An aborted test counts as an attempted test.
^ The number of attempted tests is limited to 12 per ignition cycle.
^ After a return to idle, the HO2S1 signal may stay rich or lean for several seconds and cause the test to be delayed.
^ Check for the following conditions:
^ Inspect for faulty connections at the PCM.
^ Inspect the wiring harness for damage. If the harness appears to be OK, observe the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

TEST DESCRIPTION

Steps 1-4:

Steps 5-9:

The numbers below refer to the step numbers on the diagnostic table.
2. If any component DTCs are set, diagnose those DTCs first. A fault in a component can cause the converter to appear degraded or may have caused the converter to fail.
3. Clear the DTCs in order to run the catalyst test up to 6 times this ignition cycle. The diagnostic test will abort if the A/C clutch cycles. Turn OFF the A/C. The engine must be warmed-up. The converter needs to be warmed by raising the engine speed above idle for the specified time prior to each attempted test. Determine if the DTC passed or failed this ignition cycle. If the DTC does not pass or fail, look for a possible cause.
4. This step checks the conditions that may cause the converter to appear degraded. Repair any conditions that are found before proceeding with this diagnosis.
7. If the converter must be replaced, verify that other conditions that can damage the converter are not present. Before replacing the converter, correct any possible causes of converter damage.