P0420

CIRCUIT DESCRIPTION
To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalysts ability to control emissions. The powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) located downstream of the 3-way converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst are considered to be degraded and the MIL will be illuminated if subsequent tests also indicate a failure.

CONDITIONS FOR RUNNING THE DTC
1. Meet the conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following.
- DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0401, P0403, P0404, P0405, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1374, or P1441 are not set.
- The engine has been running more than 10 minutes.
- The engine coolant temperature (ECT) is above 70°C (158°F) and below 124°C (255°F).
- The barometric pressure (BARO) is above 75 kPa.
- The vehicle is in Closed Loop.
- The intake air temperature (IAT) is above -20°C (-4°F), and is less than +100°C (+212°F).
- The battery voltage above 10.7 volts
2. Warm up the catalyst by performing the following:
2.1 Fully open the hood.
2.2 Set the transmission in park for automatic transmissions, or in neutral for manual transmissions.
2.3 Set the parking brake.
2.4 Press and hold the service brake.
2.5 Each time the engine is started the diagnostic can run up to 18 times. After the 10-minute run time and before the diagnostic runs the first time, the engine must run an additional 5 minutes between 1,500-2,500 RPM.
For any additional tests on the same key cycle, the engine speed must be between 1,500-2,500 RPM for 1 minute.
2.6 In order to activate the diagnostic return the vehicle to idle and put the vehicle in drive, or depress the clutch for a manual transmission.
3. Test the catalyst by performing the following:
3.1 Transmission is in drive for an automatic, or in neutral for manuals with clutch depress.
3.2 VIN K California Emissions-Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 6 seconds, then may go lean above 15.3 for up to 8 seconds.
3.3 VIN K FED Emissions-Within 60 seconds the air fuel ratio will go lean above 15.3 for up to 6 seconds, then may go rich below 14.1 for up to 7 seconds.
3.4 VIN 1-Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 7 seconds, then it may go lean above 15.3 for up to 9 seconds.
3.5 Verify if DTC P0420 has passed or failed this key cycle using the scan tool.

CONDITIONS FOR SETTING THE DTC
The PCM determines that the catalysts oxygen storage capacity is below an acceptable 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.

DIAGNOSTIC AIDS
Check for the following conditions:
- The PCM will not enable the catalyst test until following conditions are met:
- The engine idle speed is within 100 RPM of the desired idle.
- The throttle position (TP) is 1 percent or less.
- The short term integrator is between -20 and +20 percent.
- The catalyst test will abort if the vehicle conditions fall outside the conditions listed above.

IMPORTANT: When using a scan tool to command the cooling fans ON use the next list and previous list soft keys to enter the catalyst data list. If the catalyst data list is not entered this way the cooling fan control will be canceled.

- The catalyst test may abort due to a change in engine load (i.e., A/C, coolant fan). If this condition occurs, force the cooling fans ON with the scan tool, then return to step 2 in the table.
- The number of attempted tests is limited to 18 per key cycle.
- More than 6 tests may have to be attempted in order to get 6 completed tests. An aborted test counts as an attempted test.
- If 18 tests have been attempted, and a decision has not been made this key cycle, turn the key OFF for 30 seconds. Start the vehicle and perform the Conditions for Running the DTC, including the 10 minute engine run time.
- After returning to an idle the HO2S 1 signal may stay rich or lean for several seconds, causing the test to be delayed.
- Inspect the PCM harness connectors for backed-out terminals, improper mating, broken locks, improperly-formed or damaged terminals, and faulty terminal-to-wire connections. Use a corresponding mating terminal in order to test for proper terminal tension.
- Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on 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 1-2:

Step 3:

Steps 4-8:

Steps 9-11:

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 cause converter failure.
3. Clearing the DTCs allows the catalyst test to be run up to 18 times and completed up to 6 times this ignition cycle. If the A/C is not turned OFF the diagnostic may not run. The engine must be warm. The converter needs to be warmed by raising the engine speed above idle for the specified time prior to each attempted test. Verify if the DTC passed or failed this ignition cycle. If the DTC does not pass or fail, look for a possible reason that would cause the test to abort.
4. This step includes inspections for conditions that can cause the 3-way converter to appear degraded. Repair any problems found before proceeding with this table.