For 1990-2009 cars

### Description

In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic (TWC) converter 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 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.

### Conditions for Running the DTC

• | Barometric pressure is greater than 75 kPa.

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

• | Intake air temperature is between -10°C and 50°C (14°F to 122°F).

• | Engine speed is 2,300 to 4,500 RPM (M/T).

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

• | The engine is running in closed loop mode within 360 seconds from engine start-up.

### Conditions for Setting the DTC

• | 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 three consecutively passing trips without a fault present.

• | 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:

• | Engine misfire

• | Retarded 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 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. The catalytic converter diagnostic can be monitored on the scan tool under the MIL/System Status selection of System Information. When the Catalyst test displays a YES status (indicating that the catalytic converter diagnostic is completed) check for a DTC P0420 in the Last Test Failed screen of the scan tool. If there is no DTC P0420 displayed the Catalyst diagnostic has run and passed, indicating that no malfunction was present this time. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the system diagnostics being performed. Do not forget that the MIL/System Status tests only indicate that the test has run, not whether the test passed or failed. The Last Test Failed screen must be checked for related DTCs 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 HO2S DTCs and diagnose those DTCs first. Is there a DTC P0131, P0132, P0133, P0134, P0135, P0136, or P0141 set? | -- | Go to the Applicable DTC Table | Go to Step 3
3 |

1. Install a scan tool.
2. Start the engine.
3. Run the engine until normal operating temperature is achieved.
4. Check 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 (voltage) 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. Clear the scan tool information.
2. Operate the vehicle within the Freeze Frame conditions as specified or until the Catalyst test is complete. Refer to Test Description.

Did a DTC P0420 set?

--

Go to Step 5

Fault not Present-Go to Diagnostic Aids

5 |

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

• | Dents

• | Severe discoloration caused by excessive temperatures.

• | Holes

• | Internal rattle 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 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 toFront Pipe Replacement in Engine Exhaust.

Is the action complete?

--

Go to Step 10

--

9 |

1. Start the engine.
2. Run the engine until normal operating temperature is achieved.
3. Check 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 (voltage) 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 and check for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on scan tool?

--

Go to the Applicable DTC Table

System OK