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 HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1signal, 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:

• | There are no , EGR, ECT, MAF, TP, IAT, MAP, CMP or CKP sensor, VSS, EVAP, misfire, HO2S, IAC valve, Fuel trim, or Fuel Injector DTCs set.

• | The engine has been running longer than 10 minutes.

• | The engine coolant temperature (ECT) is between 76°C (169°F) and 124°C (255°F).

• | The BARO is above 75 kPa.

• | The vehicle is in Closed Loop.

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

• | The battery voltage above 10.7 volts.

2. Warm up the catalyst

• | Fully open hood.

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

• | 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 vehicle in drive (depress the clutch pedal for manual).

3. Test the catalyst

• | Transaxle is in drive (automatic) 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 runs and fails.

• | If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control and illuminate the TRACTION OFF lamp.

• | 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 OFF the MIL 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.

### 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 1% or less.

- | Short Term FT 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 100 RPM from desired idle.

- | Throttle Position (TP) is 1% or less.

- | Short Term FT 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 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.

• | 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. Refer to Intermittents and Poor Connections Diagnosis Connector Repairs in Wiring Systems.

• | 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. Refer to Wiring Repairs in Wiring Systems.

### 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 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 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 attempted 18 times and completed up to 6 times this ignition cycle. If the A/C is on the diagnostic may 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, leaking or plugged fuel injectors, 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 | Values | 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 may not run.

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.
6. Test may need to be completed up to 6 times in order 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 sensors for damage.

• | Oxygen sensors wiring harness for damage.

Was a problem found?

--

Go to Step 8

Go to Step 9

7 | Repair the exhaust system as necessary. Refer to Engine Exhaust. Is the action Complete? | -- | Go to Step 10 | --
8 | Repair the HO2S sensor and wiring as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 10 | --
9 | Replace the three way catalytic converter. Refer to Catalytic Converter Replacement . Notice: In order to avoid damaging the replacement three-way catalytic converter, correct the engine misfire or mechanical fault before replacing the three-way catalytic converter.

Replace the three way catalytic converter. Refer to Catalytic Converter Replacement (3.8L) .

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 may not run.

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