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 may be degraded below an acceptable threshold. If the PCM detects the degraded condition, DTC P0420 sets.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0220, P0300, P0315, P0326, P0327, P0332, P0336, P0340, P0341, P0404, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P0506, P0507, P0601, P0602, P1133, P1404, P1512, P1516 are not set. |
| • | The engine has been running for more than 10 minutes. |
| • | The engine speed is within 100 RPM of the desired idle. |
| • | The engine coolant temperature (ECT) is between 69-125°C (156-257°F). |
| • | The barometric pressure (BARO) is more than 72 kPa. |
| • | The catalytic converter (TWC) calculated temperature is between 500-700°C (932-1,292°F). |
| • | The vehicle is in Closed Loop. |
| • | The intake air temperature (IAT) is between -20 and +80°C (-4 and +176°F). |
| • | The battery voltage is more than 10.7 volts. |
| • | The throttle position (TP) is 5 percent or less. |
| • | The short term fuel trim (FT) is between -10 and +10 percent. |
| • | The engine load must be stable. |
| • | Idle the engine and keep the vehicle in Drive, or depress the clutch pedal for a manual transmission vehicle, to activate the diagnostic. Within 60 seconds the air fuel ratio will transition rich, below 14.1, for up to 7 seconds, and then may transition lean above 15.3, for up to 5 seconds. |
| • | Verify if DTC P0420 has passed or failed this ignition cycle with the scan tool. |
Conditions for Setting the DTC
The PCM determines that the efficiency of the catalyst has degraded below a calibrated 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
| • | 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 |
| - | The catalyst test may abort if the vehicle falls outside the conditions for running the DTC. |
| - | The catalyst test may abort due to a change in engine load such as the A/C or engine cooling fan cycling. |
| - | A catalyst may be temporarily degraded if a fuel with high sulfur content has been used. Drive the vehicle at highway speeds for 10 minutes and retest the converter. |
| - | If six tests have been attempted and the DTC has not run or passed during this key cycle, turn the key to OFF for 30 seconds. Perform the Conditions for Running a second time. |
| - | A maximum of 12 tests per key cycle will run if each test is a combination of pass, fail, or abort tests. |
| - | After a code clear, the catalyst test will run once if the test is a pass. |
| Important: DO NOT touch the accelerator, the HVAC, or the steering wheel while a catalyst test is in progress. |
| - | Allow the engine to return to a stabilized idle. Keep the vehicle in Drive. |
| • | 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. |
| • | 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? | 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.
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 catalyst (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: Refer to Three-Way Catalytic Converter Damage Notice in the Preface section. Replace the catalytic converter and the exhaust manifold. Refer to Catalytic Converter Replacement and Exhaust Manifold Replacement in Engine Exhaust. Did you complete the replacements? | 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 |
