Circuit Description
A 3-way catalytic converter is used in order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO present in the exhaust gas, converting these 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 three-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 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 tool catalyst data list in order to verify the following.
- Warm the catalyst.
- Test the catalyst.
| • | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0336, P0341, P0401, P0403, P0404, P0405, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1362, P1374, P1404, or P1441 are not set. |
| • | The engine has been running longer than 10 minutes. |
| • | The engine coolant temperature (ECT) is above 76°C (169°F) and below 124°C (255°F). |
| • | The barometric pressure (BARO) is above 75 kPa (10.8 psi). |
| • | The vehicle is in Closed Loop. |
| • | The intake air temperature (IAT) is above -20°C (-4°F), and is less than +100°C (+212°F). |
| • | The battery voltage is above 10.7 volts. |
| • | Fully open the hood. |
| • | Place the transmission in park (automatic) or in 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 between 1,500-2,500 RPM. |
| • | For any additional tests on the same key cycle, the engine speed must be between 1,500-2,500 RPM for 1 minute. |
| • | In order to activate the diagnostic, return the engine to idle and put the vehicle in drive for an automatic transmission, or depress the clutch for a manual transmission. |
| • | Place the transmission in drive (automatic), press the clutch and place the transmission in neutral (manua)l. |
| • | Within 60 seconds, the air/fuel ratio will go rich, below 14.1, for up to 5 seconds (6 seconds w/L67 engine), then the air/fuel ration may go lean, above 15.3, for up to 7 seconds (8 seconds w/L67 engine). |
| • | Use a scan tool in order to determine 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 an acceptable 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 PCM will NOT enable the catalyst test until the following conditions are met: |
| - | The engine idle speed is within 150 RPM of the desired idle. |
| - | The throttle position is 1.5 percent or less. |
| - | The short term integrator is between -20 percent and +20 percent. |
| • | The catalyst test will abort if the vehicle falls outside the conditions listed below while the test is running: |
| - | The engine idle speed is within 300 RPM of the desired idle, or within 150 for the w/67. |
| - | The throttle position is 1.5 percent or less. |
| - | The short term integrator is between -20 percent and +20 percent. |
| • | The catalyst test may abort due to a change in engine load (i.e., A/C, coolant fan). If this condition occurs, use the scan tool in order to force the cooling fans ON, then return to Step 2 in the table. |
| Important: When using a scan tool 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 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 condition is intermittent, refer to Intermittent Conditions .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The Diagnostic System Check-Engine Controls prompts the technician to complete some basic tests and store the Freeze Frame/Failure Records data on the scan tool if applicable.
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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.
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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.
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This step includes tests for conditions that can cause the three-way converter to appear degraded. Repair any problems found before proceeding with this table.
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If the three-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. These conditions may include: misfire, leaking or plugged fuel injectors, high engine oil or coolant consumption, retarded spark timing or weak spark. Correct any possible causes of converter damage before replacing the converter.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | ||||||||||
Are any other DTCs set? | Go to the applicable DTC tables | Go to Step 3 | |||||||||
Important: If the A/C is left ON, the diagnostic may not run. Important: If more than 6 tests have been attempted and if the DTC has not passed or failed this ignition cycle, the test may be aborted. Refer to Diagnostic Aids. Does the DTC reset? | Go to Step 4 | System OK | |||||||||
Did your inspection reveal a problem? | Go to Step 9 | Go to Step 5 | |||||||||
5 |
Did you find any of the conditions? | Go to Step 7 | Go to Step 6 | ||||||||
6 | Visually inspect for the following:
Did you find any of the conditions? | 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 | Repair as necessary. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 10 | -- | ||||||||
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Notice: A misfiring condition may damage the replacement three-way catalytic converter. Check for a misfiring condition. If a misfire is present, repair the misfiring condition before replacing the three-way catalytic converter. Replace the three-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. Important: If more than 6 tests have been attempted and if the DTC has not passed or failed this ignition cycle, the test may be aborted. Refer to Diagnostic Aids. Does the DTC reset? | Go to Step 4 | Go to Step 11 | ||||||||
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 |
