GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2000 🢒 Grand Prix 🢒 DTC 🢒 DTC P0420 Catalyst System Low Efficiency

Circuit Description

In order 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, and converts the NOx into nitrogen. The converter also has the ability to store excess oxygen, and to 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), which is located in the exhaust stream past the three-way converter. If the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. If 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 faulty. If subsequent tests also indicate a failure, DTC P0420 will set.

Conditions for Running the DTC

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

    • The following DTCs are did not set:
       - Exhaust gas recirculation (EGR) pintle position
       - Engine coolant temperature (ECT) sensor
       - Mass air flow (MAF) sensor
       - Throttle position (TP) sensor
       - Intake air temperature (IAT) sensor
       - Manifold absolute pressure (MAP) sensor
       - Crankshaft position (CKP) sensor
       - Vehicle speed sensor (VSS)
       - Evaporative emission (EVAP) system
       - Engine misfire
       - Heated oxygen (HO2) sensor
       - Camshaft position (CMP) sensor
       - Idle air control (IAC) valve
       - Fuel injector
       - Fuel Trim
    • Engine has been running longer than 10 minutes.
    • The ECT is above 76°C (169°F) and below 124°C (255°F).
    • The barometric (BARO) pressure is above 75 kPa.
    • The vehicle is in a Closed Loop.
    • The IAT is above -20°C (-4°F) and less than 100°C (212°F).
    • The battery voltage above 10.7 volts.
  1. Use the following procedure in order to warm up the catalyst:
    2. 1.1. Fully open the hood.
    1.2. Verify that the automatic transmission is in park, or that the manual transmission is in neutral.
    1.3. Set the parking brake.
    1.4. Press and hold the service brake.
    1.5. Each time the engine is started the diagnostic test 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 speed must be between 1500 RPM and 2500 RPM for 1 minute.

    1.6. To activate the diagnostic test, return the engine to an idle, and place the vehicle in drive. Depress the clutch for manual transmission.
  2. Use the following procedure in order to test the catalyst:
    3. 2.1. Place the automatic transmission in drive, or the manual transmission in neutral with the clutch depressed.
    2.2. Within 60 seconds the air fuel ratio will go rich, below 14.1 for up to 5 seconds, or for 6 seconds on the L67 engine. The engine may go lean, above 15.3 for up to 7 seconds, or for 8 seconds on the L67 engine.
    2.3. Use a scan tool in order to verify if DTC P0420 passed or failed this key cycle.

Conditions for Setting the DTC

The PCM determines that the catalysts 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
    • The PCM will not enable the catalyst test until following conditions are met:
       - The engine idle speed is plus or minus 100 RPM from desired idle.
       - The TP is 1 percent or less.
       - The short term integrator is between -20 percent and +20 percent.
    • The catalyst test will abort if the vehicle falls out side the conditions listed below while the test is running:
       - The engine idle speed is plus or minus 300 RPM (200  for w/67) from desired idle.
       - The TP is 1.5 percent or less.
       - The short term integrator is between -20 percent and +20 percent.

       Important: If you use a scan tool in order to command 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 catalyst test may abort due to a change in engine load, such as A/C, or the coolant fan. If this problem occurs use the scan tool in order to force the cooling fans ON, then return to step 2 in the table.
    • 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, turn the key OFF for 30 seconds. Start the vehicle and perform the Conditions for Running the DTC, including the 10 minute engine 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.
    • If the three-way converter needs to be replaced, verify another condition is not present which could damage the converter. Correct any possible causes of converter damage before replacing the converter. These conditions may include the following conditions:
      • Misfire conditions
      • Leaking or plugged fuel injectors
      • High engine oil or coolant consumption
      • Retarded spark timing or weak spark

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

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

  1. The Powertrain On Board Diagnostic (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.

  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.

  3. Clearing the DTCs allows the catalyst test to be tested up to 18 times and completed up to 6 times this ignition cycle. If the A/C is not turned off 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. Has the 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.

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

    5.

  5. If the three-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. Refer to Diagnostic Aids.

Step

Action

Values

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 Diagnostic Trouble Code (DTC) List/Type

Go to Step 3

3

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

  1. Turn the A/C OFF.
  2. Start the engine and wait until the vehicle is fully warmed up, at least 10 minutes, then above 1500 RPM for an additional 5 minutes. One minute for each additional attempt at 1500 RPM on same key cycle. The vehicle may be driven for the 5-minute run time.
  3. Fully open hood.
  4. Operate the vehicle within the Conditions for Running the DTC.

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

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

Does the DTC reset?

--

Go to Step 4

System OK

4

  1. Visually and physically inspect the three-way catalytic converter for damage.
  2. Inspect for the following:
    3. • Dents
    • Severe discoloration caused by excessive temperatures
    • Holes
    • Internal rattle caused by damaged catalyst substrate
  3. Verify that the three-way catalytic converter is a proper original equipment manufacturer part.

Did your inspection reveal a condition?

--

Go to Step 9

Go to Step 5

5
  1. Visually and physically inspect the exhaust system for leaks. Refer to Exhaust System Inspection in Engine Exhaust.
  2. Test the exhaust system for restrictions. Refer to Restricted Exhaust System Check .

Did you find any of the conditions?

--

Go to Step 7

Go to Step 6

6

Visually inspect for the following:

    • Oxygen sensors for damage
    • Oxygen sensors wiring harness for damage

Did you find any of the conditions?

--

Go to Step 8

Go to Step 9

7

Repair the exhaust system as necessary. Refer to Exhaust System Inspection in Exhaust Systems.

Did you complete the repair?

--

Go to Step 10

--

8

Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 10

--

9

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 in Exhaust Systems

Did you complete the replacement?

--

Go to Step 10

--

10
  1. Use a scan tool in order to clear the DTCs.

    2. 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, at least 10 minutes, then above 1500 RPM for 5 additional minutes. One minute for each additional attempt at 1500 RPM on same key cycle. The vehicle may be driven for the 5 minute run time.
  4. Fully open the hood.
  5. Operate the vehicle within the Conditions for Running the DTC.

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

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

Does the DTC reset?

--

Go to Step 11

Go to Step 4

11

Are there any additional DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List/Type

System OK