For 1990-2009 cars

### Circuit Description

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 present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it into nitrogen. The 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 heated oxygen sensor 2 located after the catalyst in order to determine whether or not 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

• | Engine coolant temperature is greater than 70°C (158°F).

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

• | 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 two 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:

• | Exhaust gas leaks. Leaks in the exhaust system can cause incorrect oxygen sensor readings.

• | HO2S 1 and HO2S 2 malfunctions. Repair the cause of any 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

• | Inadequate secondary ignition voltage

• | Excessive engine oil or engine coolant consumption

• | Rich engine fueling

An intermittent malfunction may be caused by faults in the HO2S 1 and HO2S 2 electrical circuits. Inspect the wiring harness and components for any of the following conditions:

• | Backed out terminals.

• | Improper mating of terminals.

• | Broken electrical connectors locks.

• | Improperly formed or damaged terminals

• | Faulty terminal to wire connections.

• | Physical damage to the wiring harness.

• | A broken wire inside the insulation.

• | Corrosion of electrical connections, splices, or terminals.

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

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 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 P0130, P0133, 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 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 Informatin function.
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 converter. Refer to 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 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 the 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