Circuit Description
In order to control exhaust emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust as, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it into nitrogen. The PCM has the capability to monitor this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S signal will be far less active than the signal produced by the front oxygen sensor (O2S). The PCM monitors the catalyst efficiency in two stages. When the catalyst efficiency falls below 75 percent, the PCM will set a stage 1 fail flag but does not illuminate the malfunction indicator lamp (MIL) at this point. When catalyst efficiency falls below 50 percent, the PCM will set a stage 2 fail flag and the MIL will be illuminated.
Conditions for Setting
| • | DTCs P0106, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0200, P0300-P0304, P0441, P0503, P0562, P0563, P1133, P1171 are not present. |
| • | HO2S 2 activity is excessive. |
| • | Engine speed is between 1,800-3,600 RPM. |
| • | Calculated GPS is between 11-27 gps. |
| • | Vehicle speed is between 35-80 mph. |
Action Taken
Malfunction indicator lamp (MIL) illuminates.
Conditions for Clearing the MIL/DTC
| • | The MIL will turn OFF after passing three consecutive diagnostic tests. |
| • | A history DTC will clear after 40 consecutive ignition cycles have occurred without a fault. |
| • | History and current DTCs can be cleared by using a scan tool. |
Diagnostic Aids
| • | An intermittent problem may be caused by a poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation. |
| • | Any circuitry that is suspected as causing the intermittent complaint should be thoroughly checked for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wiring connections or physical damage to the wiring harness. Refer to Intermittent Conditions . |
Test Description
The number below refers to the step number on the diagnostic table.
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If any component DTCs are set, diagnose those DTCs first. A fault in a component may cause the converter to appear degraded or may have caused it's failure.
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This step includes checks for conditions that can cause the three-way catalytic converter to appear degraded. Repair any problems found before proceeding with this chart.
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If the three-way catalytic converter needs to be replaced, make sure that another condition is not present which would caused the converter to become damaged. These conditions may include: misfire, high engine oil or coolant consumption, retarded spark timing or weak spark. To avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.
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The rear HO2S 2 will appear as an inactive sensor with a good catalyst. If the HO2S 2 activity is excessive, check for a possible exhaust leak in from of or near the sensor or for a faulty converter.
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||
1 | Was the On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | |||||
Are any component DTCs set? | -- | Go to Component DTC Charts first | Go to Step 3 | |||||
Visually/Physically check the following:
Was a problem found? | -- | Go to Step 4 | Go to Step 5 | |||||
Repair the exhaust system as necessary. Is the action complete? | -- | Go to Step 6 | -- | |||||
5 | Replace the three-way catalytic converter. Is the action complete? | -- | Go to Step 6 | -- | ||||
Does the HO2S 2 activity remain inactive after several minutes of running the engine at the specified RPM? | 1,500 RPM | Go to Step 2 | ||||||
