Circuit Description
A 3-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 the efficiency of the catalyst by first allowing the catalyst to heat, then waiting for a stabilization period while the vehicle is at a slow and stable cruise. Secondly, the ECM will transition the air-fuel ratio lean, then rich while monitoring the amplitude of the HO2S 2, then comparing this signal to a model. When the catalyst is functioning properly, the response from the HO2S 2 output signal will mirror the model. The number of test periods may vary depending on whether or not each valid test passes, fails, or aborts. Each test period may need several seconds, and may need a cumulative time of 10 minutes of driving under normal conditions for the ECM to reach a decision. If the difference between the observed HO2S 2 response and the model exceeds a calibrated threshold, the oxygen storage capability or efficiency of the catalyst may be degraded below an acceptable threshold. If the ECM detects the degraded condition DTC P0421 for bank 1 or P0431 for bank 2 sets.
Conditions for Running the DTC
| • | DTCs P0031, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0101, P0102, P0103, P0121, P0122, P0123, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0138, P0140, P0141, P0150-P0158, P0159, P0160, P0161, P0171, P0172, P0174, P0175, P0206, P0221-P0223, P0300, P0301-P0306, P0431, P0440, P0442-P0449, P0506, P0507, are not set. |
| • | The vehicle is in a Closed Loop operation. |
| • | The catalytic temperature is more than 350°C (662°F). |
| • | The engine load is between 20-45 percent. |
| • | The engine speed is between 1,000-2,120 RPM. |
| • | The mass air flow (MAF) sensor variation is less than 10 kPa. |
| • | DTCs P0421 and P0431 run once within the above enabling conditions. |
| • | This diagnostic attempts one test once the above conditions have been met for 10 minutes. This diagnostic may attempt additional tests for more than one drive cycle. |
Conditions for Setting the DTC
| • | The ECM has determined the catalyst efficiency has degraded below a calibrated threshold. |
| • | This diagnostic may conclude in as few as one test attempt. However, this diagnostic may require a cumulative time of 10 minutes, which may require multiple drive cycles. Each test attempt concludes within 1 second. |
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the 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. DO NOT touch the 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 |
| • | The Catalyst may have been temporarily contaminated with a chemical from an aftermarket fuel additive or fuels containing too much sulfur. The sulfur will coat the converter wash coat, causing the converter performance to degrade temporarily. Driving the vehicle at highway speeds for 10 minutes will burn off the contamination. Retest the converter afterwards. |
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Values | 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 catalyst monitor HO2S 2 voltage parameter transitioning below the first parameter value and above the second voltage parameter for the applicable bank? | 300 mV 650 mV | Go to Step 5 | Go to Step 4 | ||||||||
4 |
Did DTC P0421 or P0431 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 original equipment manufacturer (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 at the applicable bank 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 2 Sensor 2 . Did you complete the replacement? | -- | Go to Step 10 | -- | ||||||||
9 |
Important: Locate and repair the cause of the three-way catalytic convertor 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 |
