Description
The DTC P0420 Catalyst System Low Efficiency diagnostic monitors the operation of the catalytic converter. In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and the CO that is present in the exhaust gases. The oxidation process converts the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the heated oxygen sensors (HO2S) 1 and HO2S 2). The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gases entering the TWC converter. The HO2S 2 produces an output signal that indicates the oxygen storage capacity of the catalyst in the TWC converter. The oxygen storage capacity of the catalyst is an indicator of the converter ability to convert exhaust gases efficiently. The HO2S 1 output signal will be far more active than the HO2S 2 output signal when the converter catalyst is operating efficiently. A DTC P0420 will set if the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently.
Condition for Running the DTC
| • | The barometric pressure (BARO) is more than 75 kPa. |
| • | The engine coolant temperature is -8°C to +110°C (18-230°F). |
| • | The intake air temperature is more than -8°C (+18°F). |
| • | The fuel tank level is more than 15 percent. |
| • | The engine is operating in Closed Loop. |
Conditions for Setting the DTC
| • | The high and low voltage swing of the HO2S 2 is similar to the high and low voltage swing of the HO2S 1. |
| • | The above conditions are present for 96 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns OFF after 3 consecutively passing trips without a fault present. |
| • | A history DTC clears after 40 consecutive warm-up cycles without a fault. |
| • | Perform the scan tool Clear DTC Information function in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | Exhaust gas leaks can affect the accuracy HO2S. Refer to Exhaust Leakage . |
| • | A damaged oxygen sensor wiring harness. Inspect the HO2S 1 and HO2S 2 wiring harnesses for damage and repair as necessary. |
A TWC converter failure may be caused by any of the following conditions:
| • | Engine misfire can cause catalytic converter overheating. Refer to Symptoms - Engine Controls . |
| • | Inadequate secondary ignition voltage can cause incomplete combustion and catalytic converter overheating. Refer to Electronic Ignition (EI) System Diagnosis . |
| • | Retarded ignition timing can cause performance concerns. Refer to Ignition System Timing Adjustment . |
| Excessive engine oil or engine coolant consumption can damage the catalytic converter. Refer to Base Engine Misfire with Excessive Oil Consumption or Base Engine Misfire with Coolant Consumption . |
| • | Rich engine fueling can cause catalytic converter overheating. Refer to DTC P0172 . |
If a DTC P0420 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance (I/M) Catalyst System Set Procedure .
If a DTC P0420 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers in the diagnostic table.
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The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and to store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step verifies whether the malfunction is presently occurring.
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This step checks whether the malfunction that caused the DTC P0420 is still present. Driving the vehicle under these conditions will verify whether the fault is present. After performing the scan tool Clear DTC Information function, the DTC P0420 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0420 appears in the Not Ran Since Code Cleared list, the P0420 diagnostic has not yet run. When DTC P0420 does not appear in the Not Ran Since Code Cleared list, the P0420 diagnostic has run. If the MIL is NOT illuminated and there is no Pending DTC Status in DTC Information, the P0420 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.
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In order to avoid repeated replacement of the catalytic converter, inspect for the cause of the converter failure and repair as necessary. Refer to Diagnostic Aids.
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Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0420 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0420 appears in the Not Ran Since Code Cleared list, the DTC P0420 diagnostic has not yet run. When the DTC P0420 does not appear in the Not Ran Since Code Cleared list, the DTC P0420 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0420 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||||
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||||||
2 | Check any for any heated oxygen sensor DTCs and diagnose those DTCs first. Are the DTCs P0131, P0132, P0133, P0134, P0135, P0136, or P0141 set? | -- | Go to Step 3 | |||||||||
Is the HO2S 2 signal as active as the HO2S 1 signal? | 2,500-3,000 RPM | Go to Step 5 | Go to Step 4 | |||||||||
Did DTC P0420 set? | -- | Go to Step 5 | Go to Diagnostic Aids | |||||||||
5 |
Did you find a problem? | -- | Go to Step 8 | Go to Step 6 | ||||||||
6 |
Was a repair necessary? | -- | Go to Step 9 | Go to Step 7 | ||||||||
7 |
Was a repair necessary? | -- | Go to Step 9 | Go to Step 8 | ||||||||
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Important: Locate and repair the cause of the TWC converter failure before installing the replacement converter. Replace the three-way catalytic (TWC) converter and the warm-up three-way catalytic converter (WU-TWC), if equipped. Refer to Catalytic Converter Replacement in Engine Exhaust. Is the action complete? | -- | Go to Step 10 | -- | |||||||||
9 |
Is the HO2S 2 signal as active as the HO2S 1 signal? | 2,500-3,000 RPM | Go to Diagnostic Aids | Go to Step 10 | ||||||||
Does the DTC run and pass? | -- | Go to Step 11 | Go to Step 2 | |||||||||
11 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||||
