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: Catalyst System Low Efficiency

### Circuit/System Description

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 P0107, P0108, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0300, P0336, P0337, P0341, P0342, P0351, P0352, P0404, P0405, P0406, P0444, P0445, P0501, P0506, P0507, P0562, P1402, P1403, and P1404 are not set.

• | Before the ECM performs the idle test, the vehicle must be driven with a calculated air flow into the engine is more than 7 g/s for manual transmission or 11 g/s for automatic transmission for more than 10 seconds.

• | The transmission is in neutral or drive at idle.

• | The vehicle is operating in closed loop.

• | The engine has been running for more than 9.5 minutes.

• | The air flow into the engine is between 2.2-7.5 g/s for an automatic transmission.

• | The air flow into the engine is between 2.2-6.5 g/s for a manual transmission.

• | The throttle angle is less than 1.5 percent.

• | The EVAP purge concentration is learned.

• | The engine coolant temperature (ECT) is between 70-105°C (160-221°F).

• | The intake air temperature (IAT) is between -7 and +70°C (+19 and +158°F).

• | The long term fuel trim is learned.

• | The engine has been idling less than 60 seconds.

• | The vehicle speed is less than 3 km/h (2 mph).

• | The barometric pressure (BARO) is more than 72 kPa.

• | The catalytic converter (TWC) calculated temperature is between 500-850°C (932-1562°F).

This diagnostic attempts one test during each valid idle period once the above conditions have been met for 5 seconds. This diagnostic attempts one test for each drive cycle.

The idle test is aborted when one of the following conditions is present:

• | The A/C compressor clutch changes state.

• | The cooling fan changes state.

• | The engine speed increases 100 RPM.

• | The air/fuel ratio changes insufficiently.

### 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

• | If the condition is determined to be intermittent, refer to Testing for Intermittent Conditions and Poor Connections.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

### Circuit/System Verification

1. Ignition ON, observe the DTC information with a scan tool. DTC P0420 should not set.
2. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

1. Engine running, observe the DTC information with a scan tool. DTCs P0030, P0036, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, or P0300 should not set.

⇒ | If any DTCs are set, refer to Diagnostic System Check - 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 substrate

• | 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 HO2S

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

4. Verify the following conditions do not exist with the HO2S 2:

• | A grounded wiring harness

• | Damage

⇒ | If a condition is found, replace the HO2S 2.

5. If no physical condition is detected and the HO2S 2 is as active as the HO2S 1 after operating the engine at 1,500 RPM for 1 minute and then returning to a stabilized idle, then replace the catalytic converter.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Heated Oxygen Sensor 2 Replacement

• | Catalytic Converter Replacement