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 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

• | The following DTCs are not set: P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0201 - P0204, P0300, P0440, P0442, P0502, P0506, P0507, P0562, P0563, P0601, P1133, P1171, P1336, and P1441.

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

• | The BARO is more than 72 kPa.

• | The vehicle is in Closed Loop operation.

• | The vehicle has been driven at more than 1000 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 75°C and 125°C (167°F- 257°F).

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

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

• | The catalyst temperature is more 501°C (934°F). In order to obtain the correct temperature to run the test, drive the vehicle until the converter temperature is 100°C (180°F) above the condition for running the DTC.

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

### Conditions for Setting the DTC

The HO2S 2 response is too quick.

### Action Taken When the DTC Sets

• | The malfunction indicator lamp (MIL) will illuminate.

• | The PCM will record the operating conditions at the time that the diagnostic fails. The Freeze Frame and Failure Records buffers will store this information.

• | A history DTC stores.

### Conditions for Clearing the MIL/DTC

• | The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault.

• | A history DTC will clear after 40 consecutive warm up cycles without a fault.

• | Use a scan tool to clear the DTCs.

### 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.

An intermittent problem may be caused by the following conditions:

• | A poor electrical connection

• | A rubbed-through wire insulation

• | A broken wire inside of the insulation

Thoroughly inspect any suspected circuitry for the following conditions:

• | Any backed out terminals

• | Any incorrect mating

• | Any broken locks

• | Any incorrectly formed terminals or damaged terminals

• | Any poor terminal-to-wiring connections or physical damage to the wiring harness

Refer to Symptoms for further diagnosis.

### 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.

   These conditions may include the following items:

• | An engine misfire

• | A high engine oil or coolant consumption

• | A retarded spark timing

• | A weak spark

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

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Are any DTCs set? | -- | Go to Diagnostic Trouble Code (DTC) List/Type | 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

• | 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 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 3

System OK