Circuit Description
A three-way catalytic (TWC) converter controls 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 that are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. This HO2S 2, also referred to as the catalyst monitor sensor, produces an output signal that the PCM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The PCM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The PCM then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst is considered to be degraded below an acceptable threshold. If the PCM detects the degraded condition, DTC P0420 sets.
Conditions for Running the DTC
| • | DTCs P0030, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0404, P0405, P0442, P0443, P0455, P0496, P0502, P0503, P0506, P0507, P1133, P1134, P1351, P1352, P1361, P1362 are not set. |
| • | The engine has been running longer than 10 minutes. |
| • | The engine coolant temperature (ECT) is between 75-167°C (169-255°F). |
| • | The barometric pressure (BARO) is above 74 kPa. |
| • | The vehicle is in Closed Loop. |
| • | The intake air temperature (IAT) is between -20 to +100°C (-4 to +212°F). |
| • | The engine load must be stable. |
| • | The engine speed is within 150 RPM above or 127 RPM below the desired idle. |
| • | The throttle position (TP) is below 1.5 percent. |
| • | The short term fuel trim (FT) is between -1.5 and +1.5 percent. |
| • | The battery voltage above 10.7 volts. |
| • | The catalyst test will abort if the vehicle falls outside the conditions listed above while the test is running. |
- Warm up the catalyst.
- Test the catalyst.
| • | 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 if the tests are a combination of passes and failures. If each subsequent test is a pass the diagnostic will run a maximum of 6 times. After the 10 minute run time, and before the diagnostic runs the first time, the engine must run an additional 3 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. |
| • | The transaxle is in drive. |
| • | Within 60 seconds the air/fuel ratio will transition rich, below 13.7, for up to 7 seconds. The air/fuel ratio may then transition lean, above 16, 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
| • | These conditions may cause a catalytic converter to degrade. Inspect for the following conditions: |
| - | An engine misfire |
| - | High engine oil or high coolant consumption |
| - | Retarded spark timing |
| - | A weak or poor spark |
| - | A lean fuel mixture |
| - | A rich fuel mixture |
| - | A damaged oxygen sensor or wiring harness |
| • | If an intermittent condition cannot be duplicated, the information included in the Freeze Frame Records can be useful in determining the vehicle operating conditions when the DTC was set. |
| • | The catalyst may have been temporarily contaminated with a chemical from a fuel additive or fuel contamination. |
| • | 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. |
| • | DO NOT touch the accelerator pedal, steering wheel, or HVAC controls while a catalyst test is in progress. |
If the condition is determined to be intermittent, refer to Intermittent Conditions .
Step | Action | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | |||||||||||
2 |
Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter? | Go to Step 4 | Go to Step 3 | ||||||||||
3 |
Important: The test may need to be completed up to 6 times in order to pass or fail. 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 DTC P0420 ran and passed this ignition? | Go to Diagnostic Aids | Go to Step 4 | ||||||||||
4 |
Important: Verify that the three-way catalytic (TWC) is a high quality part that meets the original equipment manufacturer (OEM) specifications. Visually and physically inspect the catalytic converter for the following conditions:
Did you find a condition? | Go to Step 8 | Go to Step 5 | ||||||||||
5 | Visually inspect the exhaust system for the following conditions:
Did you find and correct the condition? | Go to Step 9 | Go to Step 6 | ||||||||||
6 | Visually inspect the HO2S 2 for the following conditions:
Did you find a condition? | Go to Step 7 | Go to Step 8 | ||||||||||
7 | Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | Go to Step 9 | -- | ||||||||||
8 |
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 catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust. Did you complete the replacement? | Go to Step 9 | -- | ||||||||||
9 |
Did the DTC fail this ignition? | Go to Step 2 | Go to Step 10 | ||||||||||
10 | Observe the Capture Info with a scan tool Are there any DTCs that you have not diagnosed? | System OK |
