For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC P0420: Catalyst System Low Efficiency Bank 1
DTC P0430: Catalyst System Low Efficiency Bank 2

### Circuit/System Description

Note: A new converter with less than 100 miles on it may set DTC P0420 or P0430 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

A 3-way catalytic converter (TWC) controls emissions by converting hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO2), and oxides of nitrogen (NOx) into nitrogen. The TWC also stores oxygen. When the listed idle conditions follow the listed driving conditions the Catalyst System Efficiency test will run. The engine control module (ECM) monitors this oxygen storage capacity by comparing the pre and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific idle conditions. The ECM compares the pre and post-catalyst oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.

### Conditions for Running the DTC

1. Driving Conditions

• | DTCs P0030, P0031, P0036, P0037, P0038, P0068, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0121, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0220, P0300, P0315, P0326, P0327, P0336, P0340, P0341, P0442, P0446, P0452, P0453, P0455, P0496, P0500 (manual transmission only), P0502, P0506, P0507, P0601, P0602, P0606, P0641, P0722, P0723, P1133, P1134, P1516, P1621, P2135, P2138, P2176 are not set.

• | The engine has been running for greater than 2 minutes.

• | The vehicle has been driven at greater than 1,000 RPM with the mass air flow (MAF) greater than 18 g/s for greater than 30 seconds.

• | The vehicle is in Closed Loop.

• | The vehicle has Fuel Trim Learn enabled.

• | The Engine Coolant Temperature (ECT) parameter is between 45-128°C (113-262°F).

• | The Barometric Pressure (BARO) parameter is greater than 70 kPa.

• | The Catalytic Converter Calculated Temperature parameter is greater than or equal to 420°C (788°F).

• | The Intake Air Temperature (IAT) parameter is between -20 and +85°C (-4 and +185°F).

• | The battery voltage is more than 11 volts.

2. Idle Conditions

• | The vehicle speed is less than 3.2 km/h (2 mph).

• | For automatic transmissions, the transmission is in Reverse, Drive, or Low.

• | The Throttle Position (TP) parameter is less than 2 percent.

• | The short term fuel trim (FT) is between -10 and +10 percent.

• | This diagnostic attempts one test during each valid idle period until a pass or fail decision has been reached when the above conditions have been met. This diagnostic attempts up to 8 completed tests during each drive cycle.

### Conditions for Setting the DTC

• | The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.

• | This diagnostic may conclude in one test attempt. However, this diagnostic may require as many as 18 test attempts, which would require at least 3 drive cycles. Each test attempt may conclude within approximately 1 minute.

### Action Taken When the DTC Sets

DTCs P0420 and P0430 are Type A DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0420 and P0430 are Type A DTCs.

### Reference Information

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Electrical Information Reference

Diagnostic Trouble Code (DTC) List - Vehicle

### Circuit/System Verification

1. Verify that no other DTCs are set.

⇒ | If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

2. Operate the vehicle within the Conditions for Running the DTC while observing the scan tool TWC Mon. OFF Idle Test Conditions Met parameter. The parameter will change to Yes when the OFF Idle Conditions have been met.

⇒ | If not the specified value, operate the vehicle within the OFF Idle Conditions.

3. When the parameter has changed to Yes and the conditions are safe, stop and idle the vehicle in gear.
4. Apply the service brake.
5. Monitor the scan tool TWC Monitor Test State parameter. When the parameter indicates Active, the TWC Mon Test is running.
6. Continue to monitor the parameter. When the parameter indicates Inactive, the TWC Mon Test has completed.
7. When the test has completed, the TWC Monitor Test Count parameter will increment by one and the TWC Monitor Test Result parameter will indicate Pass, Fail, or No Decision.

⇒ | If no decision has been made, repeat steps 2-6 until the Test Result parameter indicates a Pass or Fail.

### Circuit/System Testing

Caution: Refer to Heated Oxygen Sensor Resistance Learn Reset Caution in the Preface section.

Note: A new converter with less than 100 miles on it may set DTC P0420 or P0430 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

1. Inspect for the following conditions, which may cause a catalytic converter to degrade:

• | An engine misfire

• | High engine oil or high coolant consumption

• | Retarded spark timing

• | A weak or poor spark

• | A lean fuel mixture

• | A rich fuel mixture

• | A damaged oxygen sensor or wiring harness

• | The catalyst test may abort if the vehicle falls outside the conditions for running the DTC.

• | A catalyst may be temporarily degraded if a fuel with high sulfur content has been used. Drive the vehicle at highway speeds for 10 minutes and retest the converter.

2. Inspect the catalytic converter for the following conditions:

• | Dents

• | A severe discoloration caused by excessive temperatures

• | Road damage

• | An internal rattle caused by damaged catalyst substrate

• | Restrictions

⇒ | If a condition is found, replace the catalytic converter.

3. Inspect the exhaust system for the following conditions:

• | Leaks

• | Physical damage

• | Loose or missing hardware

• | Properly torqued HO2S

• | Restrictions

⇒ | If a condition is found, repair the exhaust system.

4. Inspect the post catalyst HO2S for the following conditions:

• | A grounded wiring harness

• | Damage

⇒ | If a condition is found, replace the post catalyst HO2S.

5. If all physical conditions test normal, test or replace the catalytic converter.

### Repair Instructions

Note: A new converter with less than 100 miles on it may set DTC P0420 or P0430 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

• | Exhaust Manifold Replacement - Left Side

• | Exhaust Manifold Replacement - Right Side

• | Heated Oxygen Sensor Replacement - Bank 1 Sensor 2

• | Heated Oxygen Sensor Replacement - Bank 2 Sensor 2

• | Exhaust Leakage

• | Restricted Exhaust

### Repair Verification

1. Verify that no other DTCs are set.

⇒ | If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

2. Operate the vehicle within the Conditions for Running the DTC while observing the scan tool TWC Mon. OFF Idle Test Conditions Met parameter. The parameter will change to Yes when the OFF Idle Conditions have been met.

⇒ | If not the specified value, operate the vehicle within the OFF Idle Conditions.

3. When the parameter has changed to Yes and the conditions are safe, stop and idle the vehicle in gear.
4. Apply the service brake.
5. Monitor the scan tool TWC Monitor Test State parameter. When the parameter indicates Active, the TWC Mon Test is running.
6. Continue to monitor the parameter. When the parameter indicates Inactive, the TWC Mon Test has completed.
7. When the test has completed, the TWC Monitor Test Count parameter will increment by one and the TWC Monitor Test Result parameter will indicate Pass, Fail, or No Decision.

⇒ | If no decision has been made, repeat steps 2-6 until the Test Result parameter indicates a Pass or Fail.