For 1990-2009 cars

### Circuit Description

A three-way catalytic (TWC) converter is used to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), . 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 has the ability to monitor this process using the HO2S 1 and the 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's 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 engine is running at a steady speed.

• | The engine is operating in close loop.

### Conditions for Setting the DTC

• | The engine coolant temperature is between 70°C and 110°C (158°-230°F).

• | The intake air temperature is between -10°C and +70°C (14°-158°F).

• | The barometric pressure is more than 75 kPa.

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

• | The calculated load value is between 26 percent and 80 percent.

• | The engine is running in closed loop.

• | The delay time average of HO2S 2 response is not within specifications.

• | All conditions are present for 45 seconds.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after two 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 consecutive trips during which the diagnostic runs and no fault is detected.

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

• | Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | Exhaust gas leaks

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

• | A damaged wiring harness--Inspect the HO2S 1 and 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

• | Incorrect ignition timing

• | Inadequate secondary ignition voltage

• | Excessive engine oil or engine coolant consumption

• | Rich engine fueling

If a DTC P0420 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining 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 save the applicable freeze frame data on the scan tool. This creates an electronic copy of the data that was taken when the fault occurred. The scan tool saves the information for later reference.

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

3. This step checks if the malfunction that caused the DTC P0420 is still present. The catalytic converter diagnostic can be monitored under the MIL/System Status selection of System Information on the scan tool. The Catalyst test displays a YES status in order to indicate that the catalytic converter diagnostic is complete.If the test displays a YES, check for a DTC P0420 in the Last Test Failed screen of the scan tool. If DTC P0420 is not displayed, the Catalyst diagnostic has run and passed.This indicates that a malfunction was not present this time. DTCs MUST be cleared in order to view the Current Status of the system diagnostics that are being performed. Remember that the MIL/System Status tests only indicate that the test has run.This will not indicate if the test passed or failed. The Last Test Failed screen must be checked for related DTCs in order to determine the results of the diagnostic test that was involved.

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 | Diagnose any HO2S DTCs that are set. Is there a DTC P0131, P0132, P0133, P0134, P0135, P0136, or a P0141 set? | -- | Go to the applicable DTC tables | Go to Step 3
3 |

1. Install a scan tool.
2. Start the engine.
3. Run the engine until normal operating temperature is reached.
4. Check that the engine is operating in the 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 actively equal to the HO2S 1 signal?

2,500 -3,000 RPM

Go to Step 5

Go to Step 4

4 |

1. Perform the Clear DTC Information function on the scan tool.
2. Operate the vehicle within the specified Freeze Frame conditions or until the Catalyst test is complete. Refer to Test Description.

Did a 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:

• | Dents

• | Severe discoloration that is caused by excessive temperatures.

• | Holes

• | Internal rattle that is caused by a damaged catalyst.

2. Check that the three-way catalytic converter is a high quality part and meets OEM specifications.

Was a problem found?

--

Go to Step 8

Go to Step 6

6 |

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

• | Leaks

• | Damage

• | Loose hardware 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 the wiring harness are contacting the exhaust pipe

• | A loose HO2S 2

• | Any road damage

2. Repair as necessary. Refer to Heated Oxygen Sensor Replacement .

Was a repair necessary?

--

Go to Step 9

Go to Step 8

8 | Important: Before installing the replacement converter, locate and repair the cause of the TWC converter failure. Replace the three-way catalytic converter and the warm-up three-way catalytic converter (WU-TWC), if equipped . Refer to Front Pipe Replacement in Engine Exhaust and Exhaust Manifold Replacement in Engine Mechanical--1.3L.

Is the action complete?

--

Go to Step 10

-

9 |

1. Start the engine.
2. Run the engine until normal operating temperature is reached.
3. Check that the engine is operating in the 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 actively equal to the HO2S 1 signal?

2,500 -3,000 RPM

Go to Diagnostic Aids

Go to Step 10

10 |

1. Perform the Clear DTC Information function on the scan tool, and road-test the vehicle within the Freeze Frame conditions which set the DTC.
2. Review the scan tool data, and check the data for DTCs. The repair is complete if no DTCs are displayed.

Does the scan tool display any DTCs?

--

Go to the applicable DTC table

System OK