Circuit Description
The three-way catalytic converter (TWC) reduces 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 CO that are present in the exhaust gas. This process converts these chemicals into water vapor and carbon dioxide (CO2), and will reduce the NOx, by converting them into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process using heated oxygen sensor (HO2S) bank 1 sensor 2 and HO2S bank 2 sensor 2, located in the exhaust stream after the TWC. These sensors are referred to as the catalyst monitor sensors. The catalyst monitor sensors produce an output signal that the PCM uses to indicate the oxygen storage capacity of the catalyst. This determines the catalyst's ability to effectively convert the exhaust emissions.
If the catalyst is functioning correctly, the HO2S bank 1 sensor 2 and the HO2S bank 2 sensor 2 signals will be far less active than the signals that are produced by the HO2S bank 1 sensor 1 and the HO2S bank 2 sensor 1. This indicates that the TWC oxygen storage capacity is at an acceptable threshold. When the response time of the catalyst monitor sensors are close to that of the fuel control sensors, the ability of the catalyst to store oxygen may be below an acceptable threshold.
The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs:
| • | The air-to-fuel ratio transitions from lean to rich. |
| • | The air-to-fuel ratio transitions a second time rich to lean, opposite the first air-to-fuel ratio transition. |
| • | The PCM captures the response time of the front and the rear HO2S when the air-to-fuel ratio transitions occur. The HO2S response time changes from less than 350 mV to more than 600 mV, and from more than 600 mV to less than 350 mV. |
| • | The PCM measures the time necessary for the rear HO2S voltage to cross a reference lean-to-rich threshold, and the time necessary for the front HO2S voltage to cross the same lean-to-rich threshold. The difference between the front HO2S time and the rear HO2S time indicates the oxygen storage capacity of the catalyst. If the PCM detects that this time difference is less than a predetermined value, DTC P0420 sets. |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0420 Catalyst System Low Efficiency
Conditions for Running the DTC
| • | DTCs P0030, P0036, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0121, P0125, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0155, P0171, P0172, P0174, P0175, P0201-P0208, P0300, P0325, P0327, P0336, P0341, P0342, P0343, P0351-P0358, P0404, P0410, P0442, P0443, P0446, P0449, P0455, P0491, P0492, P0496, P0502, P0503, P0506, P0507, P1133, P1134, P1153, P1154 are not set. |
| • | The engine has been running for more than 10 minutes. |
| • | The intake air temperature (IAT) is between -7°C and +200°C (+20°F and +392°F). |
| • | The barometric pressure (BARO) is more than 74 kPa (10.7 psi). |
| • | The engine coolant temperature (ECT) is between 71-120°C (160-248°F). |
| • | The predicted catalyst temperature is between 300-425°C (572-797°F). |
| • | The number of tests attempted during this ignition is less than 6. |
| • | Since the end of the last idle period, the engine speed has been more than 800 RPM for 46 seconds. |
| • | The Closed Loop fuel control is enabled. |
| • | The engine must be at a stable idle speed, within 125 RPM of desired idle. |
| • | The battery voltage is more than 10.7 volts. |
| • | A drive cycle is completed once the above conditions are met. |
| • | DTC P0420 runs continuously once the above conditions are met. |
Conditions for Setting the DTC
The PCM determines that the oxygen storage capability of the TWC has degraded to less than a calibrated threshold. If the catalyst test does not pass within the first test, then a minimum of 6 tests per drive cycle, or a maximum of 18 tests for 3 drive cycles, may occur to determine the oxygen storage capacity of the converter. Three key cycles will be necessary to complete all 3 drive cycles.
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. Do not change the engine load, ensure the A/C is OFF, the coolant fan is not cycling while the catalyst test is in progress. |
| • | Driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. |
| • | 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 Freeze Frame data 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, fuel contamination or any of the above conditions. |
| • | 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.
-
This step indicates that a catalytic converter which has been discolored may be due to an engine running rich, lean or had a previous misfire. Verifying the fuel trim percentages may be of assistance in determining if such a condition exists.
-
This step inspects for conditions that can cause the TWC efficiency to appear degraded.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||
2 | Review the DTC information on the scan tool. Are any other DTCs set? | -- | Go to Step 3 | |||||||||
3 |
Is the HO2S 2 voltage parameter transitioning below the first specified value and above the second specified value? | 350 mV 600 mV | Go to Step 5 | Go to Step 4 | ||||||||
4 |
Did DTC P0420 set? | -- | Go to Step 5 | Go to Diagnostic Aids | ||||||||
|
Important: Verify that the 3-way catalytic converter (TWC) is a high quality part that meets the OEM specifications. Visually and physically inspect the TWC for the following conditions:
Did you find and correct the condition? | -- | Go to Step 10 | 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 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 Replacement - Bank 1 Sensor 2 . Did you complete the replacement? | -- | Go to Step 10 | -- | ||||||||
9 |
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 TWC. Refer to Catalytic Converter 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 have not been diagnosed? | -- | System OK |
