Circuit Description
This diagnostic test is designed in order to measure the efficiency of the three-way catalytic converter (TWC) system. Catalytic convertor efficiency is a measure of its ability to store oxygen after converting the levels of hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) to less harmful gases. The control module is able to evaluate the catalyst efficiency once the vehicle has met the enable criteria and the vehicle is at idle instead of the steady cruise speeds used in the past.
Once the conditions for running this diagnostic trouble code (DTC) are met, the control module commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The control module issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued.
For example, if the control module were to command a rich mixture, the upstream heated oxygen sensor (HO2S) voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the more the delay before the rich or lean mixture is detected by the downstream oxygen (O2) sensor. As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2 sensor is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.
Conditions for Running the DTC
| • | No active secondary AIR DTCs |
| • | No active CMP sensor DTCs |
| • | No active ECT sensor DTCs |
| • | No active EGR DTCs |
| • | No active fuel trim DTCs |
| • | No active IAC DTCs |
| • | No active IAT sensor DTCs |
| • | No active MAF sensor DTCs |
| • | No active MAP sensor DTCs |
| • | No active O2 sensor DTCs |
| • | No active transmission DTCs |
| • | No active EVAP system DTCs |
| • | No active TP sensor DTCs |
| • | No active VS sensor DTCs |
| • | No active misfire DTCs |
| • | The engine speed is 900 RPM or greater for more than 35 seconds since last idle period |
| • | The engine has been running for at least 346 seconds and the long term fuel trim is stable |
| • | The predicted catalyst temperature is at least 475°C(887°F) |
| • | The system is in closed loop |
| • | The BARO is 73 kPa or greater |
| • | The IAT is between -7°C-117°C(20°F-167°F) |
| • | The ECT is between 75°C-117°C(167°F-243°F) |
| • | The engine has been idling for less than 60 seconds |
| • | The actual engine speed is within 100 RPM of the desired idle speed |
Conditions for Setting the DTC
The VCM determines that the oxygen storage capacity of the catalyst has degraded below a calibrated threshold.
Action Taken When the DTC Sets
The VCM will turn ON the Malfunction Indicator Lamp (MIL).
Conditions for Clearing the MIL or DTC
| • | The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed. |
| • | A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle. |
| • | Use a scan tool in order to clear the DTCs. |
Diagnostic Aids
Difficulty running the OBD II status DTC P0420 test may be encountered in areas where the test conditions cannot be maintained easily, especially in urban areas.
Test Description
The numbers below refer to the step numbers in the diagnostic table.
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This table includes checks for conditions that can cause the three-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary. Inspect the HO2S (Bank 1, Sensor 2) pigtail and engine harness for any damage that can cause an intermittent fault in HO2S (Bank 1, Sensor 2) circuits.
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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.
If the three-way catalytic converter needs to be replaced, make sure that the following conditions are not present to caused the catalyst to be damaged:
| • | Misfire |
| • | High engine oil consumption or coolant consumption |
| • | Retarded spark timing or weak spark. |
Step | Action | Value(s) | Yes | No | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This data will be lost when the Clear DTC Information function is used. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||||||||||||||||
2 |
Important: If any MAP sensor, HO2S (Bank 1, Sensor 1), (Bank 1 Sensor 2), Misfire, Fuel Trim, Vehicle Speed sensor, or ECT sensor DTCs are set, diagnose those DTCs before using this table. Are any other DTCs set? | -- | Go to The Applicable DTC Table | |||||||||||||||||||||||
Visually/physically check the following:
Was a problem found? | -- | |||||||||||||||||||||||||
4 | Check all the VCM grounds and repair as necessary. Refer to Wiring Repairs in Repair Instructions. Was a problem found? | -- | ||||||||||||||||||||||||
5 | Check all the sensor grounds and repair as necessary. Refer to Wiring Repairs in Repair Instructions. Was a problem found? | -- | ||||||||||||||||||||||||
6 | Check the H02S (Bank 1, Sensor 1) sensor high and low circuits for an intermittent open and repair as necessary. Refer to Wiring Repairs in Repair Instructions. Was a problem found? | -- | ||||||||||||||||||||||||
|
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. Important: If Three-Way Catalytic Converter needs to be replaced, check for a possible misfire (refer to DTC 300) or an engine mechanical problem. Replace the Three-Way Catalytic Converter. Is the action complete? | -- | -- | ||||||||||||||||||||||||
8 | Repair the problem as necessary. Is the repair complete? | -- | -- | |||||||||||||||||||||||
9 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||||||||||||||||||||||||
10 | Use the scan tool in order to display the Capture Info and the Review Capture Info functions. Are there any DTCs displayed that have not been diagnosed? | -- | Go to The Applicable DTC Table | System OK |
