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/h (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.

A 3-way catalytic converter (TWC) controls emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction, which oxidizes the HC and the CO that are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The engine control module (ECM) monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. The HO2S 2 produces an output signal that the ECM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The ECM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The ECM then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold.

### Conditions for Running the DTC

• | DTCs P0016 00, P0017 00, P0030 00, P0036 00, P0053 00, P0054 00, P0101 00, P0102 00, P0103 00, P0112 00, P0113 00, P0114 00, P0117 00 P0118 00, P0121 00, P0122 00, P0123 00, P0131 00, P0132 00, P0133 00, P0134 00, P013 00, P0137, P0138, P0140, P0141, P0171, P0172, P0300, P0301, P0302, P0303, P0304, P0335, P0336, P0340, P0365, P0443, P0502, P0506 00, P0507 00, P1133 00, and P2135 00 are not set.

• | Before the ECM performs the idle test, the vehicle must be driven under the following conditions:

⇒ | The engine speed is greater than 975 RPM

⇒ | The engine run time is greater than a calibrated value

⇒ | Both conditions exist for greater than 15 s

• | The throttle is in the rest position.

• | The EVAP purge concentration is learned.

• | The short term fuel trim is learned.

• | The air flow into the engine is between 1 to 10 g/s.

• | The engine has been idling less than 50 s

• | The vehicle speed is less than 2 km/h (1 MPH).

• | The engine coolant temperature (ECT) is between 46 to 140°C (115 to 284°F

• | The intake air temperature (IAT) is between -20 to +250°C (-4 to +482°F).

• | The barometric pressure (BARO) is greater than 70 kPa.

• | The catalytic converter (TWC) calculated temperature is less than 900°C (1652°F).

• | The engine is operating in closed loop.

• | Torque fuel reduction is not active.

• | The transmission is not in P/N (automatic transmission only)

• | This diagnostic attempts one test during each valid idle period once the above conditions have been met. This diagnostic attempts up to 12 tests during each drive cycle.

### Conditions for Setting the DTC

The ECM determines that the efficiency of the catalyst has degraded below a calibrated threshold.

### 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.

### Diagnostic Aids

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

### Reference Information

#### Electrical Information Reference

• | Circuit Testing

• | Testing for Intermittent Conditions and Poor Connections

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

1. Engine idling, observe the DTC information with a scan tool. Verify there are no HO2S or misfire DTCs set.

⇒ | ⇒ If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnosis the applicable DTC, before proceeding with this diagnostic procedure.

2. Verify the following conditions do not exist with the catalytic converter:

• | Dents

• | A severe discoloration caused by excessive temperatures

• | Road damage

• | An internal rattle caused by damaged catalyst subset

• | Restrictions

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

3. Verify the following conditions do not exist with the exhaust system:

• | Leaks

• | Physical damage

• | Loose or missing hardware

• | Properly torqued B52A Heated Oxygen Sensor (HO2S) or B52B Heated Oxygen Sensor (HO2S)

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

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

4. Verify the following conditions do not exist with the B52B Heated Oxygen Sensor (HO2S)

• | A grounded B52B Heated Oxygen Sensor (HO2S) wiring harness

• | Damage

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

5. If no physical condition is detected and the B52B Heated Oxygen Sensor (HO2S) is as active as the B52A Heated Oxygen Sensor (HO2S) after operating the engine at 1500 RPM for 1 min and then returning to a stabilized idle, then replace the catalytic converter.

### Repair Instructions

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.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Heated Oxygen Sensor Replacement - Sensor 2

• | Catalytic Converter Replacement

• | Exhaust Leakage

• | Symptoms - Engine Exhaust