For 1990-2009 cars

### Circuit Description

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalyst's ability to control emissions. The powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst is considered to be bad and the malfunction indicator lamp (MIL) will be illuminated if subsequent tests also indicate a failure.

### Conditions for Running the DTC

1. Meet conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following:

• | No EGR, ECT, MAF, TP, IAT, MAP, CMP or CKP Sensor, VSS, EVAP, Misfire, HO2S, IAC Valve, Fuel Trim, or Fuel Injector DTCs set.

• | Engine has been running longer than 10 minutes.

• | Engine coolant temperature (ECT) is between 76°C (169°F)-124°C (255°F).

• | BARO is above 75 kPa.

• | Vehicle is in Closed Loop.

• | Intake air temperature (IAT) is above -20°C (-4° F) and less than 100°C (212°F).

• | Battery voltage is above 10.7 volts.

2. Warm up the catalyst.

• | Fully open hood.

• | Transmission is in park, for automatics, or neutral, for manuals.

• | Set the parking brake.

• | Press and hold the service brake.

• | Each time the engine is started the diagnostic can run up to 18 times. After the 10 minute run time, and before the diagnostic runs the first time, the engine must run an additional 5 minutes between 1500 and 2500 RPM. Any additional tests on the same key cycle, the engine must be between 1500 and 2500 RPM for 1 minute.

• | To activate the diagnostic, return to idle and place the vehicle in drive, depress the clutch pedal for manual.

3. Test the catalyst.

• | Transaxle is in drive, for automatics, or neutral for manuals with clutch pedal depressed.

• | Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 7 seconds, then may go lean above 15.3 for up to 10 seconds.

• | Using the scan tool check and see if DTC P0420 has passed or failed this key cycle.

### Conditions for Setting the DTC

The PCM determines that the catalyst's oxygen storage capacity is below a threshold considered acceptable.

### Action Taken When the DTC Sets

• | The PCM will illuminate the malfunction indicator lamp (MIL) during the first trip in which the diagnostic test has been run and failed.

• | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.

• | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• | The DTC can be cleared by using a scan tool.

### Diagnostic Aids

Inspect for the following conditions:

• | The PCM will not enable the catalyst test until the following conditions are met:

- | Engine speed is plus or minus 200 RPM from desired idle.

- | Throttle Position (TP) is 1.5 percent or less.

- | Short Term FT is between -20-20 percent.

• | The catalyst test will abort if the vehicle falls outside the conditions listed below while the test is running:

- | Engine speed is plus or minus 200 RPM from desired idle.

- | Throttle position (TP) is 1.5 percent or less.

- | Short Term FT is between -20-+20 percent.

• | The catalyst test may abort due to a change in engine load. If this problem occurs use the scan tool to force the cooling fans on then return to Step 3 in the table.

| Important: When using the scan tool to force the cooling fans on use the next list and previous list soft keys to enter the catalyst data list. If the catalyst data list is not entered this way the cooling fan control will be canceled.

• | The number of attempted tests is limited to 18 per key cycle.

• | More than 6 tests may have to be attempted to get 6 completed tests. An aborted test counts as an attempted test.

• | If 18 tests have been attempted, and a decision has not been made this key cycle, then turn the key OFF for 30 seconds, start the vehicle and perform the Conditions for Running the DTC, including the 10 minute run time.

• | After returning to an idle the HO2S 1 signal may stay rich or lean for several seconds causing the test to be delayed.

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

• | Loose terminal connection

- | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

- | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.

• | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

• | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

### Test Description

Numbers below refer to the step numbers on the Diagnostic Table:

1. The powertrain OBD system check prompts the technician to complete some basic tests and store the Freeze Frame and Failure Records data on the scan tool if applicable.

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

3. Clearing the DTCs allows the catalyst test to be completed up to 6 times this ignition cycle. If the A/C is not turned OFF the diagnostic will not run. The engine must be warmed-up. The converter needs to be warmed-up by raising the engine speed above idle for the specified time prior to each attempted test. Observe the if DTC passed or failed this ignition cycle. If the DTC does not pass or fail look for a possible reason that would cause the test to abort.

4. This step includes checks for conditions that can cause the three-way converter to appear degraded. Repair any problems found before proceeding with this table.

5. If the three-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. These conditions may include: misfire, high engine oil or coolant consumption, retarded spark timing or weak spark. Correct any possible causes of converter damage before replacing the converter.

#### DTC P0420 - 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 other DTCs set? | -- | Go to the applicable DTC. | Go to Step 3
3 |

1. Clear DTCs using the scan tool.

Important: : If the A/C is left ON the following diagnostic may not run.

2. Turn the A/C OFF.
3. Start the engine and wait until the vehicle is fully warmed up.
4. Fully open hood.
5. Operate the vehicle within the conditions for running this DTC.

Important: : If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle the test may be aborting. Refer to Diagnostic Aids.

6. Test may need to be completed up to 6 times in order to pass or fail.

Does the scan tool indicate the DTC ran and passed this ignition cycle?

--

System OK

Go to Step 4

4 |

1. Visually and physically inspect the three-way catalytic converter for damage.
2. Inspect for the following:

• | Dents.

• | Severe discoloration caused by excessive temperatures.

• | Holes.

• | Internal rattle caused by damaged catalyst substrate.

3. Also, ensure that the three-way catalytic converter is a proper original equipment manufacturer part.

Did your inspection reveal a problem?

--

Go to step 9

Go to step 5

5 |

1. Visually and physically inspect the exhaust system for leaks. Refer to Exhaust Leakage - Not OBD II in Engine Exhaust.
2. If a problem is found, repair as necessary.

Did your inspection reveal a problem?

--

Go to Step 7

Go to Step 6

6 | Visually inspect for the following:

• | Oxygen sensors for damage

• | Oxygen sensors wiring harness for damage-- Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 8

Go to Step 9

7 | Repair the exhaust system as necessary. Refer to Exhaust systems Did you complete the repair? | -- | Go to Step 10 | --
8 | Repair the HO2S sensor and wiring as necessary. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 10 | --
9 | Important: : A misfiring condition may damage the replacement three-way catalytic converter. Inspect for a misfiring condition. If a misfire is present, repair the misfiring condition before replacing the three-way catalytic converter. 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 DTCs using the scan tool.

Important: : If the A/C is left on the following diagnostic will not run.

2. Turn the A/C OFF.
3. Start the engine and wait until the vehicle is fully warmed up.
4. Fully open hood.
5. Operate the vehicle within the conditions for running this DTC.

Important: : If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle the test may be aborting. Refer to diagnostic aids.

6. Test may need to be completed up to 6 times in order to pass or fail.

Does the scan tool indicate the DTC ran and passed this ignition cycle?

--

Go to Step 11

Go to Step 4

11 | Are there any additional DTCs that have not been diagnosed? | -- | Go to Applicable DTC Table | System OK