For 1990-2009 cars

### Description

In order to control emissions 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 the CO that is present in the exhaust gases. The oxidation process converts the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the heated oxygen sensors (HO2S 1 and HO2S 2). The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gases entering the TWC converter. The HO2S 2 produces an output signal that indicates the oxygen storage capacity of the catalyst in the TWC converter. The oxygen storage capacity of the catalyst is an indicator of the converter ability to convert exhaust gases efficiently. The HO2S 1 output signal will be far more active than the HO2S 2 output signal when the converter catalyst is operating efficiently. A DTC P0420 will set if the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently.

### Condition for Running the DTC

• | The barometric pressure (BARO) is more than 75 kPa.

• | The engine coolant temperature is between -8°C and +110°C (18°F-230°F).

• | The intake air temperature is more than -8°C (+18°F).

• | The fuel tank level is more than 15 percent.

• | The engine is operating in Closed Loop.

### Conditions for Setting the DTC

• | The waveform output of the HO2S 2 is similar to the waveform output of the HO2S 1.

• | The above conditions are present for 96 seconds.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns off after 3 consecutively passing trips without a fault present.

• | A history DTC clears after 40 consecutive warm-up cycles without a fault.

• | Perform the scan tool Clear DTC Information function in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | Any exhaust gas leaks

• | Any HO2S 1 and HO2S 2 malfunctions. Repair all HO2S DTCs before diagnosing DTC P0420.

• | A damaged wiring harness. Inspect the HO2S 1 and the HO2S 2 wiring harnesses for damage and repair as necessary.

A TWC converter failure may be caused by any of the following conditions:

• | An engine misfire

• | A retarded ignition timing

• | An inadequate secondary ignition voltage

• | An excessive engine oil or engine coolant consumption

• | A rich engine fueling

If a DTC P0420 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0420 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0420 appears in the Not Ran Since Code Cleared list, the P0420 diagnostic has not yet run. When the DTC P0420 does not appear in the Not Ran Since Code Cleared list, the P0420 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the P0420 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.

If a DTC P0420 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.

### Test Description

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

1. The OBD System Check prompts the technician to complete some basic checks and to store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step verifies whether the malfunction is presently occurring.

3. This step checks whether the malfunction that caused the DTC P0420 is still present. Driving the vehicle under these conditions will verify whether the fault is present. After performing the scan tool Clear DTC Information function, the DTC P0420 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0420 appears in the Not Ran Since Code Cleared list, the P0420 diagnostic has not yet run. When DTC P0420 does not appear in the Not Ran Since Code Cleared list, the P0420 diagnostic has run. If the MIL is NOT illuminated and there is no Pending DTC Status in DTC Information, the P0420 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.

4. In order to avoid repeated replacement of the catalytic converter, inspect for the cause of the converter failure and repair as necessary. Refer to Diagnostic Aids.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Check any for any heated oxygen sensor DTCs and diagnose those DTCs first. Are the DTCs P0131, P0132, P0133, P0134, P0135, P0136, or P0141 set? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Step 3
3 |

1. Install a scan tool.
2. Start the engine.
3. Run the engine until the normal operating temperature is reached.
4. Verify that the engine is operating in Closed Loop.
5. Run the engine at the specified value for 3 minutes.
6. Monitor the HO2S 1 and the HO2S 2 signals on the scan tool.

Is the HO2S 2 signal as active as the HO2S 1 signal?

2,500-3,000 RPM

Go to Step 5

Go to Step 4

4 |

1. Perform the scan tool Clear DTC Information function.
2. Operate the vehicle within the Freeze Frame conditions as specified or until the P0420 diagnostic test has run. Refer to the Test Description.

Did DTC P0420 set?

--

Go to Step 5

Go to Diagnostic Aids

5 |

1. Visually and physically inspect the three-way catalytic converter for the following concerns:

• | Any dents

• | Any severe discoloration caused by excessive temperatures

• | Any holes

• | An internal rattle caused by a damaged catalyst

2. Verify that the 3-way catalytic converter is a high quality part that meets the OEM specifications.

Did you find a problem?

--

Go to Step 8

Go to Step 6

6 |

1. Visually and physically inspect the exhaust system for any of the following conditions:

• | Any leaks

• | Any damage

• | Any loose or missing hardware

2. Repair as necessary.

Was a repair necessary?

--

Go to Step 9

Go to Step 7

7 |

1. Visually and physically inspect the HO2S 2 for the following conditions:

• | The pigtail and wiring harness making contact with exhaust pipe

• | A loose HO2S 2

• | Any road damage

2. Repair as necessary.

Was a repair necessary?

--

Go to Step 9

Go to Step 8

8 | Important: Locate and repair the cause of the TWC converter failure before installing the replacement converter. Replace the three-way catalytic (TWC) converter and the warm-up three-way catalytic converter (WU-TWC), if equipped. Refer to Catalytic Converter Replacement in Engine Exhaust.

Is the action complete?

--

Go to Step 10

--

9 |

1. Start the engine.
2. Run the engine until the normal operating temperature is reached.
3. Verify that the engine is operating in Closed Loop.
4. Run the engine at the specified value for 3 minutes.
5. Monitor the HO2S 1 and the HO2S 2 signals on the scan tool.

Is the HO2S 2 signal as active as the HO2S 1 signal?

2,500-3,000 RPM

Go to Diagnostic Aids

Go to Step 10

10 |

1. Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC.
2. Review the scan tool data.
3. Check for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on scan tool?

--

Go to Diagnostic Trouble Code (DTC) List

System OK