GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Savana 🢒 DTC 🢒 DTC P0420 or P0430

Circuit Description

This diagnostic test is designed in order to measure the efficiency of the three-way catalytic converter (TWC) system. Catalytic convertor efficiency is a measure of its ability to store oxygen after converting the levels of hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) to less harmful gases. The powertrain control module (PCM) is able to evaluate the catalyst efficiency once the vehicle has met the enable criteria and the vehicle is at idle instead of the steady cruise speeds used in the past.

Once the conditions for running this diagnostic trouble code (DTC) are met, the control module commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The control module issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued.

For example, if the control module were to command a rich mixture, the upstream heated oxygen sensor (HO2S) voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the more the delay before the rich or lean mixture is detected by the downstream oxygen (O2) sensor. As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2 sensor is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.

Conditions for Running the DTC
    • DTCs P0101, P0103, P0106, P0108, P0112, P0113, P0117, P0118, P0125, P0131, P0135, P0137, P0138, P0140, P0141, P0151, P0155, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0200, P0300, P0305, P0325, P0327, P0335, P0336, P0341, P0343, P0351, P0358, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1120, P1125, P1133, P1134, P1153, P1154, P1220, P1221, P1275, P1276, P1280, P1281, P1285, P1286, P1441, P1514, P1515, P1516, P1517, or P1518 are not set.
    • The engine speed is at least 1,100 RPM for more than 44 seconds since the last idle period.
    • The engine has been running for at least 360 seconds and the long term FT is stable.
    • The predicted catalyst temperature is more than 390°C (887°F).
    • The system is in Closed Loop.
    • The barometric pressure (BARO) is at least 73 kPa.
    • The intake air temperature (IAT) is more than -7 to +117°C (20-167°F).
    • The engine coolant temperature (ECT) is between 75-117°C (167-243°F).
    • The engine has been idling for less than 120 seconds.
    • The actual engine speed is within 100 RPM of the desired idle speed.

Conditions for Setting the DTC

The PCM determines that the oxygen storage capacity of the catalyst has degraded below a calibrated threshold.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

The use of fuel with a high sulfur or lead content may degrade a marginal converters performance. Be sure to check fuel quality.

An intermittent may be caused by any of the following conditions:

    • A poor connection
    • Rubbed through wire insulation
    • A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

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

  1. This table checks for conditions that can cause the three-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary.

    2.

  2. Before the three-way catalytic converter is replaced, make sure that the following conditions are not present:

    3. • Misfire
    • High engine oil consumption or coolant consumption
    • Retarded spark timing or weak spark

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Check the exhaust system for the following:
    2. • Leaks
    • Loose or missing hardware
  2. Repair as necessary. Refer to Symptoms - Engine Exhaust in Engine Exhaust.

Did you find a problem?

Go to Step 5

Go to Step 4

4
  1. Verify that the correct original equipment three-way catalytic converter is installed.
  2. Check the converter for the following:
    3. • Dents
    • Severe discoloration
    • Holes
  3. Ensure that the oxygen sensors are properly installed and that the wiring connections are properly retained and not damaged.
  4. Repair as necessary. Refer to Symptoms - Engine Exhaust in Engine Exhaust.

Did you find a problem?

Go to Step 5

Go to Step 6

5
  1. Using the scan tool, clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the engine reaches normal operating temperature.
  4. Select DTC and the Specific DTC function.
  5. Enter the DTC number that was set.
  6. Operate the vehicle, within the Conditions for Setting this DTC, until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic Passed?

System OK

Go to Step 6

6

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 catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Is the action complete?

System OK

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