For 1990-2009 cars

### Circuit Description

A 3-way catalytic converter is used in order to control the 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 present in the exhaust gas. This converts the HC and the CO into a harmless water vapor and a carbon dioxide, and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also has the ability to store oxygen. The powertrain control module (PCM) has the capability to monitor this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past 3-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust emissions effectively. The PCM monitors the catalyst efficiency by first allowing the catalyst to heat up, waiting for a stabilization period while the engine is idling. Then, the PCM adds and removes the 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 O2S 1. When the HO2S 2 response is close to that of the O2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be deficient and the MIL will illuminate.

### Conditions for Running the DTC

• | DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0204, P0300, P0325, P0336, P0340, P0341, P0440, P0442, P0446, P0452, P0453, P0480, P0481, P0502, P0503, P0506, P0507, P0601, P0602, P1133, P1171, P1336, P1441, or P1621 are not set.

• | The engine has been running more than 520 seconds after the throttle has moved.

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

• | The vehicle is in closed loop operation.

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

• | The battery voltage is more than 9 volts.

Then, with the vehicle stopped:

• | The engine is idling at less than 150 RPM from the desired idle.

• | The vehicle is in drive.

• | The throttle position (TP) is 0 percent.

• | The engine coolant temperature (ECT) is between 69-125°C (156-257°F).

• | The intake air temperature (IAT) is between -20 to +80°C (-4 to +176°F).

• | The short term (ST) fuel trim is between 93-163 (-28 percent and +28 percent).

• | The predicted catalyst temperature is between 495-603°C (923-1,117.5°F).

• | The above conditions are present for up to 30 seconds.

### 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. Do not change the engine load while a catalyst test is in progress.

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

- | An engine misfire

- | A high engine oil or coolant consumption

- | A retarded spark timing

- | A weak spark

- | A lean fuel system

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. If any component DTCs are set, diagnose those DTCs first. A malfunction in a component can cause the converter to appear degraded or may have caused the malfunction.

2. This step includes inspects for conditions that can cause the 3-way catalytic converter to appear degraded.

3. If you need to replace the 3-way catalytic converter, make sure that another condition is not present which could damage the converter. Correct any possible causes of converter damage before replacing the catalytic converter.

#### DTC P0420 - Three Way Catalyst (TWC) System Low Efficiency

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 | Are any DTCs set? | Go to Diagnostic Trouble Code (DTC) List | Go to Step 3
3 |

1. Use a scan tool to clear the DTCs.
2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 4

Go to Diagnostic Aids

4 |

1. Turn OFF the ignition.
2. Visually and physically inspect the following components for leaks:

• | The exhaust system--Refer to Exhaust Leakage in Engine Exhaust.

• | The rear heated oxygen sensor (HO2S 2)

3. If a problem is found, repair as necessary.

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5 | Replace the 3-way catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust. Did you complete the repair? | Go to Step 6 | --
6 |

1. Use a scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

Go to Step 7

Go to Step 3

7 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | Go to Diagnostic Trouble Code (DTC) List | System OK