To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalysts ability to control emissions. The powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) located downstream of the 3-way converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst are considered to be degraded and the MIL will be illuminated if subsequent tests also indicate a failure.

1. Meet conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following.

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DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0401, P0403, P0404, P0405, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1374, P1441 are not set.

•	The engine has been running more than 10 minutes.

•	The engine coolant temperature (ECT) is 70-124°C (158-255°F).

•	The barometer (BARO) is above 75 kPa (10.8 psi).

•	The vehicle is in Closed Loop.

•	The intake air temperature (IAT) is between -20 to 100°C (-4 to 212°F).

•	The battery voltage is above 10.7 volts

2. Warm up the catalyst by performing the following:

2.1.	Fully open the hood.

2.2.	Set the transmission in park for automatic transmissions, or in neutral for manual transmissions.

2.3.	Set the parking brake.

2.4.	Press and hold the service brake.

2.5.	Each time the engine is started the diagnostic can run up to 18 times. After the 10-minute run time and before the diagnostic runs the first time, the engine must run an additional 5 minutes between 1500-2500 RPM.

Any additional tests on the same key cycle, the engine speed must be between 1500-2500 RPM for 1 minute.

2.6.	In order to activate the diagnostic, return the engine to idle and put the vehicle in drive, or depress the clutch for a manual transmission.

3. Test the catalyst by performing the following:

3.1.	Transmission is in drive for an automatic, or in neutral for manuals with clutch depress.

3.2.	VIN K California Emissions-- Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 6 seconds. Then the air-fuel ratio may go lean above 15.3 for up to 8 seconds.

3.3.	VIN K FED Emissions--Within 60 seconds the air fuel ratio will go lean above 15.3 for up to 6 seconds, then may go rich below 14.1 for up to 7 seconds.

3.4.	VIN 1 -- Within 60 seconds, the air fuel ratio will go rich below 14.1 for up to 7 seconds. Then the air-fuel ratio may go lean above 15.3 for up to 9 seconds.

3.5.	Verify if DTC P0420 has passed or failed this key cycle with the scan tool.

The PCM determines that the catalyst oxygen storage capacity is below an acceptable threshold.

Inspect for the following conditions:

The numbers below refer to the step numbers on the diagnostic table.

2. If any component DTCs are set, diagnose those DTCs first. A fault in a component can cause the converter to appear degraded or may cause converter failure.

3. Clearing the DTCs allows the catalyst test to be run up to 18 times and completed up to 6 times this ignition cycle. If the A/C is not turned OFF the diagnostic may not run. The engine must be warm. The converter needs to be warmed by raising the engine speed above idle for the specified time prior to each attempted test. Verify if the DTC passed or failed this ignition cycle. If the DTC does not pass or fail, look for a possible reason that would cause the test to abort.

4. This step includes inspections for conditions that can cause the 3-way converter to appear degraded. Repair any problems found before proceeding with this table.