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
| • | Meet the conditions for engine warm up. |
| • | DTCs P0101-P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0125, P0130-P0135, P0137, P0138, P0140, P0141, P0150-P0155, P0171, P0172, P0174, P0175, P0201-P0206, P0300, P0325, P0336, P0341, P0401, P0403-P0405, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1120, P1125, P1133, P1134, P1153, P1154,P1220, P1221, P1336, P1352, P1361, P1362, P1441, or P1523 are not set. |
| • | The engine has been running longer than 10 minutes. |
| • | The engine coolant temperature (ECT) is more than 70°C (158°F), and less than 124°C (255°F). |
| • | The barometric pressure (BARO) is more than 75 kPa. |
| • | The vehicle is in Closed Loop. |
| • | The intake air temperature (IAT) is more than -20°C (-4°F), and less than 100°C (212°F). |
| • | The battery voltage more than 10.7 volts. |
| • | Warm the catalyst. |
| - | Fully open the hood. |
| - | Place the transmission in Park for an automatic transmission, or in Neutral for a manual transmission. |
| - | 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 1,500-2,000 RPM. For any additional tests on the same key cycle, the RPM must be between 1,500-2,000 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. |
| • | Test the catalyst. |
| - | Place the transmission in Drive for an automatic transmission, or in Neutral and depress the clutch for a manual transmission. |
| - | Within 60 seconds, the air/fuel ratio will go rich, below 14.0, for up to 5 seconds, then the air/fuel ratio may go lean, above 15.3, for up to 8 seconds. |
| - | Use the scan tool in order to check if DTC P0420 has passed or failed this key cycle |
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
| • | 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
Inspect for the following conditions:
| • | The PCM will NOT enable the catalyst test until following conditions are met: |
| - | The engine speed is plus or minus 150 RPM from the desired idle. |
| - | The throttle position (TP) is less than 0.5 percent. |
| - | The short term integrator is between -20 to +20 percent. |
| • | The catalyst test will ABORT if the vehicle falls outside the conditions listed below while the test is running: |
| - | The engine speed is plus or minus 150 RPM from the desired idle. |
| - | The throttle position (TP) is less than 0.5 percent. |
| - | The short term integrator is between -20 to +20 percent. |
| • | The catalyst test may abort due to a change in engine load due to the A/C, the coolant fan, or other components. If this condition occurs, use the scan tool in order to force the cooling fans on. Return to step 3 in the table, then repeat the test. |
| Important: When using the scan tool in order to force the cooling fans ON, use the next list and use the previous list soft keys in order 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. |
| • | You may need to attempt more than 6 tests in order to complete 6 tests. An aborted test counts as an attempted test. |
| • | If 18 tests have been attempted, and if a decision has not been made during this key cycle, turn OFF the key for 30 seconds. Start the engine, and perform the Conditions for Running the DTC, including the 10-minute 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 to store the Freeze Frame/Failure Records on the scan tool, if applicable.
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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.
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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.
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This step includes tests for conditions that can cause the 3-way converter to appear degraded. Repair any conditions before proceeding with this table.
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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.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | ||||||||||
Are any other DTCs set? | Go to applicable DTC first | Go to Step 3 | |||||||||
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Important:: If the A/C is left on the following diagnostic may not run.
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. Does the scan tool indicate the DTC ran and passed this key cycle? | System OK | Go to Step 4 | |||||||||
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:
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 Replacement - Bank 1 Sensor 1 , Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 , Heated Oxygen Sensor Replacement - Bank 1 Sensor 2 or Wiring Repairs in Wiring Systems. Did you complete the replacement? | Go to Step 10 | -- | ||||||||
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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.
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. 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 |
