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

Circuit Description

To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-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 the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx 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 catalysts ability to control emissions. The PCM monitors this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the 3-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 are considered to be degraded and the MIL will be illuminated if subsequent tests also indicate a failure.

Conditions for Running the DTC
  1. Meet the conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following:
    2. • None of the following DTCs are set:
       - Exhaust gas recirculation (EGR) system
       - Engine coolant temperature (ECT) sensor
       - Mass air flow (MAF) sensor
       - Throttle position (TP) sensor
       - Intake air temperature (IAT) sensor
       - Manifold absolute pressure (MAP) sensor
       - Camshaft position (CMP) sensor
       - Crankshaft position (CKP) sensor
       - Vehicle speed sensor (VSS)
       - Evaporative emissions (EVAP) system
       - Misfire
       - Heated oxygen sensor (HO2S)
       - Idle air control (IAC) valve
       - Fuel trim
       - Fuel injector
    • The engine has been running for more than 10 minutes.
    • The engine coolant temperature is more than 76°C (169°F) and less than 124°C (255°F).
    • The BARO is more than 75 kPa.
    • The vehicle is in Closed Loop.
    • The intake air temperature is more than -20°C (-4°F) and less than 100°C (212°F).
    • The battery voltage is more than 10.7 volts.
  2. Warm up the catalyst.
    3. • Fully open the hood.
    • Ensure the 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 at between 1500 and 2500 RPM.
    • For any additional tests on the same key cycle, the engine speed must be between 1500 RPM and 2500 RPM for 1 minute.
    • To activate the diagnostic, return to idle and put the vehicle in drive or depress the clutch.
  3. Test the catalyst.
    4. • Ensure the transmission is in drive (automatic) or neutral (manual).
    • Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 7 seconds, and then may go lean above 15.3 for up to 10 seconds.
    • Determine if DTC P0420 has passed or failed this key cycle using the scan tool.

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

Check for the following conditions:

    • The PCM will not enable the catalyst test until the following conditions are met:
       - The engine idle speed is within 100 RPM of the desired idle.
       - The throttle position (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 conditions fall outside the conditions listed below while the test is running:
       - The engine idle speed is within 125 RPM of the desired idle.
       - The throttle position (TP) is 1 percent or less.
       - The short term integrator is between -20 percent and +20 percent .

       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 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 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 in order 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.
    • Inspect the PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper terminal tension.
    • Inspect the wiring harness for damage. If the harness appears to be OK, observe the display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

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 checks 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 caused converter failure.

    3.

  3. Clearing the DTCs allows the catalyst test to be run 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. 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.

  4. This step includes checks for conditions that can cause the 3-way converter to appear degraded. Repair any problems found before proceeding with this table.

    5.

  5. If the 3-way converter needs to be replaced, ensure another condition is not present which could damage the converter. Such conditions may include: misfire, leaking or plugged fuel injectors, high engine oil or coolant consumption, retarded spark timing, or weak spark. Correct any possible cause 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

If any DTCs are stored, save the Freeze Frame and Fail Records information using the scan tool.

Are any other DTCs set?

--

Go to Powertrain Control Module Diagnosis

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. Clear the DTCs.
  3. Start the engine and wait until the vehicle is fully warmed up for at least 10 minutes.
  4. Run the engine above 1500 RPM for an additional 5 minutes. (One minute for each additional attempt at 1500 RPM on the same key cycle). The vehicle may be driven for the 5 minute run time.
  5. Fully open the hood.
  6. Operate the vehicle within the Conditions for Running the DTC.

    7. 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.

  7. The 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 3-way catalytic converter for damage.
  2. Check for the following:
    3. • Dents
    • Severe discoloration caused by excessive temperatures
    • Holes
    • Internal rattles caused by damaged catalyst substrate
  3. Also, ensure that the 3-way catalytic converter is the correct original equipment 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, restrictions, or damage.
  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:

    • Damaged oxygen sensors
    • Damage to the HO2S wiring harness

Was a problem found?

--

Go to Step 8

Go to Step 9

7

Repair the exhaust system as necessary. Refer to Restricted Exhaust System Check .

Did you complete the repair?

--

Go to Step 10

--

8

Repair the HO2S sensor and wiring as necessary. Refer to Heated Oxygen Sensor 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 3-way catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Did you complete the repair?

--

Go to Step 10

--

10
  1. Clear the DTCs using the scan tool.

    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 for at least 10 minutes.
  4. Run the engine 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.
  5. Fully open the hood.
  6. Operate the vehicle within the Conditions for Running the DTC.

    7. 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.

  7. The 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 any additional DTCs set that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List/Type

System OK