Circuit Description
A 3-way catalytic (TWC) converter is used in order to control the 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 a harmless water vapor and carbon dioxide, and will reduce the NOx, converting the NOx into nitrogen. The TWC converter also has the ability to store oxygen. The powertrain control module (PCM) has the capability to monitor this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past the TWC 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 PCM monitors the catalyst efficiency by first allowing the catalyst to heat, waiting for a stabilization period while the engine is idling. Then, the PCM adds and removes the 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 O2S 1. When the HO2S 2 response is close to that of the O2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be bad and the malfunction indicator lamp (MIL) will illuminate.
Conditions for Running the DTC
| • | DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0204, P0300, P0301-P0304, P0325, P0336, P0340, P0341, P0440, P0442, P0446, P0452, P0453, P0480, P0502, P0503, P0506, P0507, P0601, P0602, P1133, P1336, P1441, or P1621 are not set. |
| • | The engine has been running more than 530 seconds after the throttle has moved. |
| • | The barometric pressure (BARO) is more than 72 kPa. |
| • | The vehicle is in a Closed Loop operation. |
| • | The vehicle has been driven at more than 1,000 RPM for more than 35 seconds if the vehicle has an automatic transmission. |
| • | The vehicle has been driven at more than 1,000 RPM for more than 45 seconds if the vehicle has a manual transmission. |
| • | The battery voltage is more than 9 volts. |
With the vehicle stopped:
| • | The engine is idling less than 150 RPM from the desired idle. |
| • | The vehicle is in drive, with an automatic transmission, or in neutral, with a manual transmission. |
| • | The throttle position (TP) is 0 percent. |
| • | The engine coolant temperature (ECT) is more than 69°C (156°F), and less than 125°C (257°F). |
| • | The intake air temperature (IAT) is more than -20°C (-4°F), and less than 80°C (176°F). |
| • | The short term (ST) fuel trim is between -28 to +28. |
| • | The catalytic temperature is more than 510°C (950°F). |
| • | The above conditions are present for up to 30 seconds. |
Conditions for Setting the DTC
The PCM 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. Do not change the engine load, that is, do not reset the A/C, the coolant fan, or the heater motor 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 system |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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If any component DTCs are set, diagnose those DTCs first. A malfunction in a component can cause the converter to appear degraded, or may have caused the malfunction.
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This step includes checks for conditions that can cause the TWC converter to appear degraded. Before you proceed with this table, repair any conditions that you find.
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If you need to replace the TWC converter, ensure that another condition which could cause damage to the converter is not present. Correct any possible causes of converter damage before replacing the TWC converter in order to avoid damage to the replacement converter.
Step | Action | Yes | No | ||||
|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | |||||
Were any component DTCs set? | Go to Step 3 | ||||||
Did you find a condition? | Go to Step 4 | Go to Step 5 | |||||
4 | Repair the exhaust system as necessary. Is the action complete? | Go to Step 6 | -- | ||||
Replace the TWC converter. Refer to Catalytic Converter Replacement in Engine Exhaust. Is the action complete? | Go to Step 6 | -- | |||||
6 |
Does the DTC run and pass? | Go to Step 7 | Go to Step 2 | ||||
7 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | System OK |
