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 Descriptor

DTC P0420 00: Catalyst System Low Efficiency

### Circuit/System Description

Note: A new converter with less than 160 km (100 mi) on it may set DTC P0420 00 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 h may correct the condition.

A 3-way catalytic converter controls emissions by converting hydrocarbons and carbon monoxide (CO) into carbon dioxide (CO2), and oxides of nitrogen (NOx) into nitrogen. The TWC also stores oxygen. When the listed driving conditions are met, 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 driving 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

• | DTCs P000A 00, P000B 00, P0010 00, P0013 00, P0016 00, P0017 00, P0030 00, P0031 00, P0032 00, P0036 00, P0038 00, P0117 00, P0118 00, P0119 00, P0121 00, P0122 00, P0123 00, P0131 00, P0132 00, P0133 00, P0134 00, P0135 00, P0136 00, P0137 00, P0138 00, P0139 00, P0140 00, P0141 00, P0171 00, P0201 00, P0202 00, P0203 00, P0204 00, P0221 00, P0222 00, P0223 00, P0261 00, P0262 00, P0264 00, P0265 00, P0267 00, P0268 00, P0270 00, P0271 00, P0301 00, P0302 00, P0303 00, P0304 00, P0335 00, P0336 00, P0340 00, P0365 00, P0366 00, P0443 00, P0446 00, P0458 00, P0459 00, P0496 00, P0562 00, P0563 00, P0642 00, P0643 00, P0652 00, P0653 00, P0661 00, P0662 00, P2088 00, P2089 00, P2090 00, P2091 00, P2096 00, P2097 00, P2100 00, P2101 00, P2122 00, P2123 00, P2127 00, P2128 00, P2176 00, P2297 00, P2301 00, P2303 00, P2304 00, P2306 00, P2307 00, P2309 00, P2310 00, P2A00 00, P2A0 001 are not set.

• | The engine speed is between 1040-2760 RPM.

• | The engine load is between 15-50%.

• | The vehicle is in closed loop.

• | The calculated exhaust mass gas flow is between 5-55.56 g/s, and stable.

• | The calculated catalyst temperature is between 400-850°C (852-1562°F), and stable.

• | The rear HO2S has exceeded the dew point for more than 60 s.

• | This diagnostic attempts one test during each period when the above conditions have been met. This diagnostic attempts up to 4 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 6 test attempts which will require at least 2 drive cycles. However, this diagnostic may require as many as 8 test attempts. Depending on driving conditions, more than 2 drive cycles may be required. Each test attempt may conclude within approximately 1 min

### Action Taken When the DTC Sets

DTC P0420 is a Type A DTC.

### Conditions for Clearing the DTC

DTC P0420 is a Type A DTC.

### Reference Information

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

• | Heated Oxygen Sensor Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

Caution: Refer to Three-Way Catalytic Converter Damage Caution in the Preface section.

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

Note: A new converter with less than 160 km (100 miles) on it may set DTC P0420 00 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 h may correct the condition.

1. If any other DTCs are set perform those diagnostics first.
2. 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 min and retest the converter.

3. Operate the vehicle within the conditions for running the DTC.

⇒ | If the DTC fails this key cycle, continue with this procedure.

4. 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.

5. Inspect the exhaust system for the following conditions:

• | Leaks

• | Physical damage

• | Loose or missing hardware

• | Properly torqued B52A Heated Oxygen Sensor (HO2S)

• | Restrictions

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

6. Inspect the B52B Heated Oxygen Sensor (HO2S) for the following conditions:

• | A grounded wiring harness

• | Damage

⇒ | If a condition is found, replace the B52B Heated Oxygen Sensor (HO2S).

⇒ | If no physical condition is detected, replace the catalytic converter.

### Repair Instructions

Note: A new converter with less than 160 km (100 mi) on it may set DTC P0420 00 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 h may correct the condition.

• | Catalytic Converter Replacement

• | Heated Oxygen Sensor Replacement - Sensor 2

### Repair Verification

1. Clear the DTCs.
2. Operate the vehicle within the Conditions for Running and Setting for at least 10 min.
3. Stop the vehicle.
4. Engine OFF, allow the ECM to power down.
5. Operate the vehicle within the Conditions for Running and Setting for at least 10 min. DTC P0420 00 should run and pass.