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 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 HO2S1 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 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 VSS sensor, Tp sensor, EVAP, HO2S sensor, misfire, IAT sensor, MAP sensor, IAC sensor, Fuel trim, EGR, ECT sensor, MAF sensor, or CKP sensor DTC's set.

• | Engine has been running longer than 10 minutes.

• | Engine coolant temperature (ECT) is above 76°C (168°F).

• | Baro is above 75 kPa.

• | Vehicle is in Closed Loop.

• | Intake air temperature (IAT) is above °-29C (-20 F) and less than 100°C (212°F).

2. Warm up the catalyst.

• | Fully open hood.

• | Transmission is in park (automatic) or neutral (manual).

• | Set the parking brake.

• | Engine speed is greater than 1,500 RPM for 1 minute.

• | Return to Idle.

3. Test the catalyst.

• | Transmission is in drive (automatic) or neutral (manual).

• | With in 5-20 seconds the air fuel ratio will go lean above 15.3 for up to 7 seconds, then it may go rich below 14.1 for up to 7 seconds.

• | Using the scan tool check and see if DTC P0420 has passed or failed this ignition 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 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 Fail Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been 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 the scan tool Clear Info function or by disconnecting the PCM battery feed.

### Diagnostic Aids

Check for the following conditions:

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

- | Engine speed is plus or minus 100 RPM from desired idle.

- | Throttle Position (TP) is 0%.

- | Short term integrator is between -20% and +20%.

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

- | Engine speed is plus or minus 125 RPM from desired idle.

- | Throttle Position (TP) is 0%.

- | Short term integrator is between -20% and +20%.

• | The catalyst test may abort due to a change in engine load (i.e., A/C, coolant fan). If this problem occurs use the scan tool to force the cooling fans on then repeat the test.

| 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.

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

• | The number of attempt tests is limited to 18 per ignition cycle.

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

• | Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

• | Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table:

1. The powertrain OBD system check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable.

2. If any component DTCs are set, diagnose those DTC's 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. Check and see 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 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | 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.
2. Turn the A/C OFF.

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

3. Start the engine and wait until the vehical is fully warmed up.
4. Fully open hood.
5. Operate the vehical within the conditions for running this DTC.
6. Test may need to be completed up to 6 times inorder to pass or fail.

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.

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. Check 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.
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 check for the following:

• | Oxygen sensor(s) for damage.

• | Oxygen sensor(s) wiring harness for damage.

Was a problem found?

--

Go to Step 8

Go to Step 9

7 | Repair the exhaust system as necessary. Is action Complete? | -- | Go to Step 10 | --
8 | Repair the HO2S sensor and wiring as necessary. Is action complete? | -- | Go to Step 10 | --
9 | Notice: A misfiring condition may damage the replacement three-way catalytic converter. Check 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.

Is action complete?

--

Go to Step 10

--

10 |

1. Clear DTCs using the scan tool.
2. Turn the A/C OFF.

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

3. Start the engine and wait until the vehical is fully warmed up.
4. Fully open hood.
5. Operate the vehical within the conditions for running this DTC.
6. Test may need to be completed up to 6 times inorder to pass or fail.

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.

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