For 1990-2009 cars

### Circuit Description

In order 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 the CO present in the exhaust gas, converting these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx, converting these chemicals into nitrogen. The converter also has the ability to store excess oxygen and to release the stored oxygen in order to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalyst's ability to control emissions. The powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) located in the exhaust stream past 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 is considered to be bad and the malfunction indicator lamp (MIL) will be illuminated if subsequent tests also indicate a failure.

### Conditions for Running the DTC

1. The conditions for engine warm-up have been met.

Use the scan tool catalyst data list in order to verify the following:

• | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0325, P0327, P0336, P0341, P0401, P0403, P0404, P0405, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1362, P1404, or P1441 are not set.

• | No exhaust gas recirculation (EGR), vehicle speed sensor (VSS), evaporative emission (EVAP) sensor, misfire, PRNDL, Trans, secondary air injection (AIR), or idle air control (IAC) valve DTCs set.

• | The engine has been running longer than 10 minutes.

• | The engine coolant temperature (ECT) is between 70-124°C (158-255°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 is above 10.7 volts.

2. Warm the catalyst.

• | Fully open the hood.

• | Set the transmission in park for automatic transmissions, or in neutral for manual transmissions.

• | Set the parking brake.

• | Press and hold the service brake.

• | 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 must be between 1,500-2,500 RPM for 1 minute.

• | In order to activate the diagnostic, return the engine to idle, and place the automatic transmission in drive, or depress the clutch pedal for a manual transmission.

3. Test the catalyst.

• | Place the transaxle in drive for automatic transmissions, or in neutral with the clutch pedal depressed for manual transmissions.

• | Within 60 seconds, the air/fuel ratio will go lean, and should be above 15.3 for up to 7 seconds. The air/fuel ratio may then go rich, below 14.1 for up to 8 seconds.

• | Use the scan tool to check if DTC P0420 has passed or failed this key cycle.

### Conditions for Setting the DTC

The PCM determines that the catalyst's 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

Inspect for the following conditions:

• | The PCM will not enable the catalyst test until following conditions are met:

- | The engine speed is plus or minus 305 RPM from the desired idle.

- | The throttle position (TP) is 1.5 percent or less.

- | The short term fuel trim (FT) is between -20 percent 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 within 305 RPM from the desired idle.

- | The TP is 1.5 percent or less.

- | The short term FT is between -20 percent and +20 percent.

• | The catalyst test may abort due to a change in engine load from the A/C or from the coolant fan. If this condition occurs, use the scan tool in order to force the cooling fans ON, then return to Step 3 in the table.

| Important: : When you use the scan tool in order to force the cooling fans ON, use the next list and the previous list soft keys in order to enter the catalyst data list. If the catalyst data list is not entered this way, the cooling fan control will be canceled.

• | The number of attempted tests is limited to 18 per key cycle.

• | You may need to attempt more than 6 tests in order to get 6 completed tests. An aborted test counts as an attempted test.

• | If 18 tests have been attempted, and if 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 run time.

• | After returning to an idle, the HO2S 1 signal may stay rich or lean for several seconds and cause the test to be delayed.

If the condition is intermittent, refer to Intermittent Conditions .

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 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 its failure.

2. Clearing the DTCs allows the catalyst test to be completed up to 6 times during this ignition cycle. If the A/C is not turned OFF the diagnostic will 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. Check if the DTC passed or failed this ignition cycle. If the DTC does not pass or fail, look for a reason that would cause the test to abort.

3. This step includes tests for conditions that can cause the 3-way converter to appear degraded. Repair any problems before proceeding with this table.

4. If the 3-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. These conditions may include: misfire, high engine oil or coolant consumption, retarded spark timing, weak spark, or a rich or lean fuel system. Correct any possible causes of converter damage before replacing the converter.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Are any other DTCs set? | Go to the applicable DTC | Go to Step 3
3 |

1. Clear DTCs using the scan tool.

Important: If the A/C is left ON the following diagnostic may not run.

2. Turn OFF the A/C.
3. Start the engine, and wait until the vehicle is fully warm.
4. Fully open the hood.
5. Operate the vehicle within the Conditions for Running this DTC.

Important: If more than 6 tests have been attempted, and if the DTC has not passed or failed this ignition cycle, the test may be aborting. Refer to Diagnostic Aids.

6. The test may need to be completed up to 6 times in order to pass or fail.

Does the scan tool indicate the DTC ran and passed this ignition cycle?

System OK

Go to Step 4

4 |

1. Inspect the catalytic converter for damage. Refer to Restricted Exhaust in Engine Exhaust.
2. Inspect for the following:

• | Dents

• | Severe discoloration caused by excessive temperatures

• | Holes

• | Internal rattle caused by damaged catalyst substrate

3. Ensure that the catalytic converter is the correct original equipment manufacturer part.

Did your inspection reveal a problem?

Go to Step 9

Go to Step 5

5 | Inspect the exhaust system for leaks. Refer to Restricted Exhaust in Engine Exhaust. Did your inspection reveal a problem? | Go to Step 7 | Go to Step 6
6 | Inspect for the following conditions:

• | Damaged oxygen sensors

• | A damaged oxygen sensor wiring harness--Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

Go to Step 8

Go to Step 9

7 | Repair the exhaust system as necessary. Refer to Symptoms - Engine Exhaust in Engine Exhaust. Did you complete the repair? | Go to Step 10 | --
8 | Replace the HO2S sensor or repair harness wiring as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | Go to Step 10 | --
9 | Notice: A misfiring condition may damage the replacement three-way catalytic converter. Check for a misfiring condition. If a misfire is present, repair the misfiring condition before replacing the three-way catalytic converter. Replace the catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Did you complete the replacement?

Go to Step 10

--

10 |

1. Clear DTCs using the scan tool.

Important: If the A/C is left ON, the following diagnostic may not run.

2. Turn OFF the A/C.
3. Start the engine and wait until the vehicle is fully warm.
4. Fully open the hood.
5. Operate the vehicle within the conditions for running this DTC.

Important: If more than 6 tests have been attempted and if the DTC has not passed or failed, this ignition cycle the test may be aborting. Refer to Diagnostic Aids.

6. The test may need to be completed up to 6 times in order to pass or fail.

Does the scan tool indicate the DTC ran and passed this ignition cycle?

Go to Step 11

Go to Step 4

11 | Are there any additional DTCs that have not been diagnosed? | Go to the applicable DTCs | System OK