For 1990-2009 cars

### Description

The DTC P0420 Catalyst System Low Efficiency diagnostic monitors the operation of the catalytic converter. 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 (PCM) 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 more than 75 kPa.

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

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

• | Fuel tank level is more than 15 percent.

• | Engine is operating in closed loop.

• | Engine is running at a steady speed between 1,500 and 3,500 RPM (A/T only).

• | Engine is running at a steady speed between 2,750 and 3,750 RPM (M/T only).

### Conditions for Setting the DTC

• | The high and low voltage swing of the HO2S 2 is similar to the high and low voltage swing of the HO2S 1.

• | Above conditions are present for 96 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 can affect the accuracy HO2S. Refer to Exhaust Leakage in Engine Exhaust.

• | A damaged oxygen sensor 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 can cause catalytic converter overheating. Refer to Symptoms - Engine Controls .

• | Inadequate secondary ignition voltage can cause incomplete combustion and catalytic converter overheating. Refer to Electronic Ignition (EI) System Diagnosis .

• | Excessive engine oil or engine coolant consumption can damage the catalytic converter. Refer to Base Engine Misfire with Excessive Oil Consumption or Base Engine Misfire with Coolant Consumption in Engine Mechanical-1.6L.

• | Rich engine fueling can cause catalytic converter overheating. Refer to DTC P0172 .

If a DTC P0420 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance (I/M) Catalyst System Set Procedure .

If you cannot duplicate a DTC P0420, 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 Diagnostic System Check-Engine Controls 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. The scan tool can monitor the catalytic converter diagnostic 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 Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
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 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. 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. Perform the scan tool Clear DTC Information function.
2. Operate the vehicle within the Freeze Frame conditions as specified or until the DTC P0420 diagnostic is complete.

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 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.

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:

• | 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 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. 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. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 11

Go to Step 2

11 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK