For 1990-2009 cars

### Circuit Description

The DTC P0420 Catalyst System Low Efficiency diagnostic monitors the operation of the exhaust system catalytic converter. In order to control exhaust emission of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx), a three-way catalyst converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx into nitrogen. The powertrain control module (PCM) has the capability to monitor this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way catalytic converter. The PCM compares the signal from the heated oxygen sensor 1 (HO2S 1), located before the catalyst with the sensor 2 (HO2S 2) located after the catalyst in order to determine if the catalyst has deteriorated. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than the HO2S 1. Both HO2S signals changing at similar rates indicates that the catalyst is deteriorating.

### Conditions for Running the DTC

• | The engine coolant temperature (ECT) is more than 70°C (158°F).

• | The engine speed is between 2,500-3,000 RPM for at least 3 minutes while driving at certain vehicle speeds.

• | The engine is operating in Closed Loop.

### Conditions for Setting the DTC

The HO2S 2 signal is as active as HO2S 1 signal.

### 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 stores the conditions which were present when the DTC set as Freeze Frame data.

### Conditions for Clearing the MIL/DTC

• | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.

• | The DTC clears after 40 consecutive warm-up cycles have occurred without a fault.

• | A DTC can be cleared by using the scan tool Clear DTC Information function.

### Diagnostic Aids

Check for any of the following conditions:

• | A contaminated or malfunctioning HO2S can cause inaccurate sensor readings. For checks of the HO2S refer to Diagnostic Aids in DTC P0130 .

• | Observe the HO2S 1 voltage and fuel trim readings with a scan tool. If the HO2S 1 voltage is fixed low, while the fuel trim is constantly high, check the mass air flow (MAF) sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition. This lean fueling condition can cause the HO2S 2 voltage to be fixed low, causing the DTC P0420 diagnostic to fail.

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

• | 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 .

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

• | An engine misfire

• | An inadequate secondary ignition voltage

• | Any excessive engine oil or engine coolant consumption

• | A rich engine fueling

An intermittent malfunction may be caused by faults in the HO2S electrical circuits. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

### Test Description

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

1. The Diagnostic System Check- Engine Controls 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 I/M System Information. When the Catalyst test displays a YES status, indicating that the catalytic converter diagnostic is completed, check for 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 I/M System Information tests only indicate that the test has run, not if 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.

Step | Action | 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. Are DTCs P0130, P0133, 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 normal operating temperature is reached.
4. Check that the engine is operating in Closed Loop.
5. Run the engine at 2,500-3,000 RPM 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?

Go to Step 5

Go to Step 4

4 |

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 or until the Catalyst test is complete. Review the supporting text.

Did a DTC P0420 set?

Go to Step 5

Go to Diagnostic Aids

5 |

1. Visually and physically inspect the 3-way catalytic converter for the following concerns:

• | Any dents

• | A severe discoloration caused by excessive temperatures

• | Any holes

• | An internal rattle caused by a damaged catalyst

2. Check that the 3-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:

• | 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 the wiring harness making contact with the 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 catalytic converter failure before installing the replacement converter. Replace the 3-way catalytic converter. 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 normal operating temperature is reached.
3. Check that the engine is operating in Closed Loop.
4. Run the engine at 2,500-3,000 RPM 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?

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 or until the Catalyst test is complete. Review 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