Circuit Description
A three-way catalytic (TWC) converter controls exhaust 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 is present in the exhaust gas. This process will convert the HC and the CO into water vapor and carbon dioxide (CO2), and will reduce the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The engine control module (ECM) monitors this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past the three-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. The ECM monitors the catalyst efficiency by first allowing the catalyst to heat, then waiting for a stabilization period while the engine is idling. Then, the ECM adds and removes fuel while monitoring the reaction of the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the HO2S 1. When the HO2S 2 response is close to that of the HO2S 1, the oxygen storage capability or efficiency of the catalyst is may be degraded below an acceptable threshold. If the ECM detects the degraded condition DTC P0420 sets.
Conditions for Running the DTC
| • | DTCs P0068, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0220, P0300, P0326, P0327, P0336, P0340, P0341, P0442, P0446, P0452, P0453, P0496, P0500, P0506, P0507, P0601, P0602, P0603, P0604, P0606, P0641, P0651, P0700, P0701, P1133, P1134, P1137, P1138, P1516, P1680, P1681, P2120, P2125, P2135, P2138, P2176 are not set. |
| • | The engine has been running more than 9 minutes after the throttle has moved. |
| • | The vehicle is in a Closed Loop operation. |
| • | The vehicle has been driven at more than 1,000 RPM for more than 35 seconds. |
| • | The air flow parameter is less than 8 g/s. |
| • | The ECT is between 70-125°C (158-257°F). |
| • | The IAT is between -21 and +125°C (-6 and +176°F). |
| • | The barometric pressure is more than 72 kPa. |
| • | The ignition voltage is more than 9 volts. |
| • | The catalytic converter (TWC) calculated temperature is between 600-675°C (1,112-1,247°F). |
| • | The vehicle speed is less than 5 km/h (3 mph). |
| • | The engine must be at idle, and between -75 and +150 RPM of the desired speed. |
This DTC P0420 is sampled once at an idle when the above conditions are met. If the first test fails, the ECM will take more samples during the current ignition cycle or may take samples for up to six ignition cycles. The extra sampling is to ensure that a converter has failed, not just temporarily poisoned.
Conditions for Setting the DTC
The ECM has determined the catalyst efficiency 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
The catalyst test may abort due to a change in the engine load. Allow the engine to return to a stabilized idle. An automatic transmission must remain in drive. A manual transmission is shifted into neutral. Do not touch the accelerator pedal, steering wheel or HVAC controls while a catalyst test is in progress.
Certain conditions may cause a catalytic converter to degrade. These conditions may include the following conditions:
| • | An engine misfire |
| • | High engine oil or high coolant consumption |
| • | Retarded spark timing |
| • | A weak spark |
| • | A lean fuel mixture |
| • | A rich fuel mixture |
| • | A damaged oxygen sensor wiring harness |
| • | 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 twelve 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. |
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | |||||||||
2 | Observe the DTC information on the scan tool. Are there any other component DTCs set? | Go to Step 3 | |||||||||
3 |
Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter? | Go to Step 5 | Go to Step 4 | ||||||||
4 |
Did DTC P0420 set? | Go to Step 5 | Go to Diagnostic Aids | ||||||||
5 |
Important: Verify that the three-way catalytic converter is a high quality part that meets the OEM specifications. Visually and physically inspect the three-way catalytic converter for the following conditions:
Did you find a condition? | Go to Step 9 | Go to Step 6 | ||||||||
Visually inspect the exhaust system for the following conditions:
Did you find and correct the condition? | Go to Step 10 | Go to Step 7 | |||||||||
7 | Visually inspect the catalyst monitor HO2S 2 for the following conditions:
Did you find a condition? | Go to Step 8 | Go to Step 9 | ||||||||
8 | Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | Go to Step 10 | -- | ||||||||
9 |
Important: Locate and repair the cause of the three-way catalytic converter failure before installing the replacement converter. Refer to Diagnostic Aids. Replace the three-way catalytic converter. Refer to Exhaust Manifold Pipe Replacement in Engine Exhaust. Did you complete the replacement? | Go to Step 10 | -- | ||||||||
10 |
Did the DTC fail this ignition? | Go to Step 2 | Go to Step 11 | ||||||||
11 | Observe the Capture Info with a scan tool. Are there any DTCs that you have not diagnosed? | System OK |
