2001 Chevrolet/Geo Camaro  

DTC P0420

Circuit Description

In order 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 the CO present in the exhaust gas, converting the chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, 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 powertrain control module (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 is considered to be bad and the malfunction indicator lamp (MIL) will be illuminated if subsequent tests also indicate a failure.

Conditions for Running the DTC
Conditions for Setting the DTC

The PCM determines that the catalyst's oxygen storage capacity is less than an acceptable threshold.

Action Taken When the DTC Sets
Conditions for Clearing the MIL/DTC
Diagnostic Aids

Inspect for the following conditions:

If the condition is intermittent, refer to Intermittent Conditions .

Test Description

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

  1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic tests and to store the Freeze Frame/Failure Records on the scan tool, if applicable.

  2. If any other DTCs are set, diagnose those DTCs first. A fault in a component can cause the converter to appear degraded, or may have caused the failure.

  3. Clearing the DTCs allows the catalyst test to be attempted 18 times, and completed up to 6 times during this key cycle. If the A/C is ON, the diagnostic may not run. The engine must be warmed. The converter needs to be warmed by raising the engine speed more than idle for the specified time prior to each attempted test. Check if the DTC passed or failed this key cycle. If the DTC does not pass or fail, look for a condition that could cause the test to abort.

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

  5. If the 3-way converter needs to be replaced, ensure that another condition which could damage the converter is not present. Conditions may include, misfire, high engine oil or coolant consumption, retarded spark timing, weak spark, or plugged or leaking injectors. Correct any possible causes of converter damage before replacing the converter.

DTC P0420 - TWC System Low Efficiency

Step

Action

Yes

No

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 applicable DTC first

Go to Step 3

3

Important:

If the A/C is left on the following diagnostic may not run.

  1. With a scan tool, refer to and record Failure Records and clear DTCs.
  2. Turn the A/C OFF.
  3. Fully open the hood.
  4. Operate the vehicle within the Failure Record conditions.

    Important

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

  5. 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 key cycle?

System OK

Go to Step 4

4
  1. Visually and physically inspect the three-way catalytic converter for damage.
  2. Inspect for the following:
    • Dents
    • Severe discoloration caused by excessive temperatures
    • Holes
    • Internal rattle caused by damaged catalyst substrate
  3. Also, ensure that the 3-way catalytic converter is a proper original equipment manufacturer part.

Did you find damage?

Go to Step 9

Go to Step 5

5

Visually and physically inspect the exhaust system for leaks. Refer to Exhaust Leakage in Engine Exhaust.

Did you find a condition?

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 a condition?

Go to Step 8

Go to Step 9

7

Repair the exhaust system as necessary.

Did you complete the repair?

Go to Step 10

--

8

Replace the oxygen sensor or repair the wiring as necessary. Refer to Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 , Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 , Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 or Wiring Repairs in Wiring Systems.

Did you complete the replacement?

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 replacement?

Go to Step 10

--

10

Important

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

  1. With a scan tool, clear the DTCs.
  2. Turn OFF the A/C.
  3. Fully open the hood.
  4. Operate the vehicle within the Failure Record conditions.

    Important

    If more than 6 tests have been attempted and the DTC has not passed or failed this key cycle the test may be aborting. Go to Diagnostic Aids.

  5. 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 key cycle?

Go to Step 11

Go to Step 4

11

Does the scan tool indicate any DTCs that you have not diagnosed?

Go to applicable DTC

System OK