Circuit Description
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 which oxidizes the HC and 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 TWC 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 TWC. 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. Use the scan tool catalyst data list in order to verify the following conditions:
- Warm up the catalyst.
- Test the catalyst.
| • | DTCs P0030, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0404, P0405, P0410, P0440, P0442, P0443, P0502, P0503, P0506, P0507, P1133, P1134, P1351, P1352, P1361, P1362, or P1441 are not set. |
| • | The engine has been running longer than 10 minutes. |
| • | The engine coolant temperature (ECT) is between 76-124°C (169-255°F). |
| • | The barometric pressure (BARO) is above 75 kPa. |
| • | The vehicle is in Closed Loop. |
| • | The intake air temperature (IAT) is between -20 to +100°C (-4 to +212°F). |
| • | The battery voltage above 10.7 volts. |
| • | Fully open the hood. |
| • | The transmission is in Park, automatic, or 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 must be between 1,500-2,500 RPM for 1 minute. |
| • | In order to activate the diagnostic, return the engine to idle and place the vehicle in drive, or depress the clutch pedal for a manual transmission. |
| • | The transaxle is in drive, for automatic transaxles, or neutral for manual transaxles with the clutch pedal depressed. |
| • | Within 60 seconds the air/fuel ratio will go lean, below 14.1, for up to 7 seconds. The air/fuel ratio may then go rich, above 15.3, for up to 7 seconds. |
| • | Using the scan tool check and see 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 below a threshold considered acceptable.
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 the following conditions are met: |
| - | The engine speed is plus or minus 265 RPM from the desired idle. |
| - | The throttle position (TP) is 1.5 percent or less. |
| - | The short term FT is between -20 to +20 percent. |
| • | The catalyst test will abort if the vehicle falls outside the conditions listed above while the test is running. |
| Important: : When you use the scan tool to force the cooling fans ON, use the next list and 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 catalyst test may abort due to a change in engine load, i.e., A/C, coolant fan. If this problem occurs, use the scan tool to force the cooling fans ON then return to step 3 in the table. |
| • | The number of attempted tests is limited to 18 per key cycle. |
| • | More than 6 tests may have to be attempted in order 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 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 determined to be intermittent, refer to Intermittent Conditions .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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If any component DTCs are set, diagnose those DTCs first. A fault in a component can cause the TWC to appear degraded or may have caused its failure.
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Clearing the DTCs allows the catalyst test to be 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 TWC needs to be warmed-up by raising the engine speed above idle for the specified time prior to each attempted test. Inspect and see if 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 TWC to appear degraded. Repair any problems found before proceeding with this table.
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If the TWC needs to be replaced, make sure that another condition is not present which could damage the TWC. These conditions may include misfire, high engine oil or coolant consumption, retarded spark timing, weak spark, or a rich or lean fuel system. Correct any possible causes of TWC damage before replacing the TWC.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | |||||||||
Are any other DTCs set? | Go to Step 3 | ||||||||||
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 ignition cycle, the test may be aborting. Refer to Diagnostic Aids. Does the scan tool indicate the DTC ran and passed this ignition cycle? | System OK | Go to Step 4 | |||||||||
Did your inspection reveal a problem? | 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 your inspection reveal a problem? | Go to Step 7 | Go to Step 6 | ||||||||
6 | Visually inspect the following components for damage:
Was a repair necessary? | 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 the HO2S sensor and wiring as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | 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 catalytic converter (TWC). 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 ignition cycle, the test may be aborting. Refer to Diagnostic Aids. Does the scan tool indicate the DTC ran and passed this ignition cycle? | Go to Step 11 | Go to Step 4 | ||||||||
11 | Are there any additional DTCs that you have not diagnosed? | System OK |
