Circuit Description
A three-way catalytic converter (TWC) 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 TWC. 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. 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 may be degraded below an acceptable threshold. If the ECM detects the degraded condition DTC P0420 sets.
Conditions for Running the DTC
| • | DTCs P0031, P0037, P0038, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0300, P0326, P0327, P0336, P0340, P0341, P0500 (manual transmission only), P0506, P0507, P0601, P0602, P0606, P0641, P0722, P0723, P1133, P1134 are not set. |
| • | The engine has been running more than 9.5 minutes after the throttle has moved. |
| • | The vehicle is in a Closed Loop operation. |
| • | The vehicle has been driven at more than 1,200 RPM for more than 30 seconds. |
| • | The Short Term Fuel Trim (FT) is between -5 and +5 percent. |
| • | The catalytic temperature is more than 375°C (707°F), and less than 750°C (1,382°F). |
| • | An automatic transmission must remain in drive. |
| • | The vehicle speed is less than 1.6 km/h (1 mph). |
| • | The engine must be at a stabilized idle. |
| • | The manifold absolute pressure (MAP) sensor variation is less than 6 kPa. |
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 |
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 for the applicable bank? | 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 (TWC) is a high quality part that meets the OEM specifications. Visually and physically inspect the TWC 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 TWC failure before installing the replacement converter. Refer to Diagnostic Aids. 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 you have not diagnosed? | System OK |
