For 1990-2009 cars

### Circuit Description

A three-way catalytic (TWC) converter controls exhaust 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 is present in the exhaust gas. This process will convert the HC and the CO into water vapor and carbon dioxide (CO2), and will reduce the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past the three-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. The PCM monitors the catalyst efficiency by first allowing the catalyst to heat, then waiting for a stabilization period while the engine is idling. Then, the PCM adds and removes fuel while monitoring the reaction of the HO2S. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the HO2S 1. When the HO2S 2 response is close to that of the HO2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be degraded below an acceptable threshold. If the PCM detects the degraded condition DTC P0420 sets.

### Conditions for Running the DTC

• | DTCs P0105, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0204, P0217, P0300, P0301-P0304, P0326, P0327, P0336, P0340, P0341, P0440, P0442, P0446, P0452, P0453, P0480, P0481, P0500 (manual transmission only), P0502, P0503, P0506, P0507, P0562, P0563, P0601, P0602, P1133, P1137, P1138, P1171, P1336, P1441, P1621, P2135 are not set.

• | The engine has been running more than 10 minutes after the throttle has moved.

• | The vehicle is in a Closed Loop operation.

• | The vehicle has been driven at more than 1,200 RPM for more than 45 seconds.

• | The calculated catalytic temperature is between 375--750°C (707--1,392°F).

• | The calculated air flow parameter is less than 8 g/s.

• | The ECT is between 70--125°C (158--257°F).

• | The IAT is between -21 and +125°C (-6 and 176°F).

• | The barometric pressure is more than 72 kPa.

• | The ignition voltage is more than 9 volts

• | The engine must be at idle, and within -75 to +150 RPM of desired idle speed.

This DTC P0420 is sampled once at an idle when the above conditions are met. If the first test fails, the PCM will take more samples during the current ignition cycle or may take samples for up to six ignition cycles. The extra sampling is to ensure that a converter has failed, and has not been temporarily poisoned.

### Conditions for Setting the DTC

The PCM has determined the catalyst efficiency has degraded below a calibrated threshold.

### Action Taken When the DTC Sets

• | The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

• | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

### Conditions for Clearing the MIL/DTC

• | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• | A current DTC, Last Test Failed, clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• | Clear the MIL and the DTC with a scan tool.

### Diagnostic Aids

The catalyst test may abort due to a change in the engine load. Allow the engine to return to a stabilized idle. An automatic transmission must remain in drive. A manual transmission is shifted into neutral. DO NOT touch the accelerator pedal, steering wheel, or HVAC controls while a catalyst test is in progress.

Certain conditions may cause a catalytic converter to degrade. These conditions may include the following conditions:

• | An engine misfire

• | High engine oil or high coolant consumption

• | Retarded spark timing

• | A weak spark

• | A lean fuel mixture

• | A rich fuel mixture

• | A damaged oxygen sensor wiring harness

### Test Description

The number below refers to the step number on the diagnostic table.

1. This step includes inspections for conditions that can cause the three-way catalytic converter to appear degraded. Before you proceed with this table, repair any conditions that you find.

Step | Action | Yes | No
--- | --- | --- | ---
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Observe the DTC information on the scan tool. Are there any other component DTCs set? | Go to Diagnostic Trouble Code (DTC) List | Go to Step 3
3 |

1. Allow the engine to reach operating temperature.
2. Ensure Closed Loop operation is achieved.
3. Increase the engine speed to 1500 RPM for 1 minute.
4. Return the engine to a stabilized idle.
5. Monitor the HO2S 1 and the HO2S 2 for the applicable bank.

Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter for the applicable bank?

Go to Step 5

Go to Step 4

4 |

1. Clear the DTCs with a scan tool.
2. Start the engine.
3. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did DTC P0420 set?

Go to Step 5

Go to Diagnostic Aids

5 | Important: Verify that the three-way catalytic converter is a high quality part that meets the OEM specifications. Visually and physically inspect the three-way catalytic converter for the following conditions:

• | Dents or damage

• | Severe discoloration caused by excessive temperatures

• | Holes or punctures

• | Internal rattles caused by a damaged catalyst element

Did you find a condition?

Go to Step 9

Go to Step 6

6 | Visually inspect the exhaust system for the following conditions:

• | Restrictions--Refer to Restricted Exhaust and Leaks-- Exhaust Leakage in Engine Exhaust.

• | Physical damage

• | Loose or missing hardware

• | The HO2S 2 for proper torque

Did you find and correct the condition?

Go to Step 10

Go to Step 7

7 | Visually inspect the catalyst monitor HO2S 2 for the following conditions:

• | The pigtail and wiring harness making contact with the exhaust or any good ground

• | Road damage

Did you find a condition?

Go to Step 8

Go to Step 9

8 | Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | Go to Step 10 | --
9 | Important: Locate and repair the cause of the three-way catalytic convertor failure before installing the replacement converter. Refer to Diagnostic Aids. Replace the three-way catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Did you complete the replacement?

Go to Step 10

--

10 |

1. Clear the DTCs with a scan tool.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 11

11 | Observe the Capture Info with a scan tool. Are there any DTCs that you have not diagnosed? | Go to Diagnostic Trouble Code (DTC) List | System OK