To control 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 gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it 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 in the exhaust stream past the three-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 is considered to be degraded and the malfunction indicator lamp (MIL) will illuminate 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, or P1441 are not set.

•	The engine has been running more than 10 minutes.

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

•	The barometric pressure (BARO) is above 75 kPa.

•	The vehicle is in Closed Loop.

•	The intake air temperature (IAT) is above -20°C (-4°F) and less than 100°C (212°F).

•	The battery voltage above 10.7 volts

2. Warm up the catalyst.

2.1.	Fully open the hood.

2.2.	Place the automatic transmission in park, or place the manual transmission in neutral while you depress the clutch.

2.3.	Set the parking brake.

2.4.	Press and hold the service brake.

2.5.	Each time the engine is started the diagnostic test can run up to 18 times. After the 10-minute run time and before the diagnostic test 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.	To activate the diagnostic test, return the engine to idle, and put the vehicle in drive. For a manual transmission, depress clutch.

3. Test the catalyst.

•	Place the automatic transmission in park, or place the manual transmission in neutral while you depress the clutch.

•	VIN K California Emissions-- Within 60 seconds the air fuel ratio (A/F) will go rich below 14.1 for up to 6 seconds, then the A/F may go lean above 15.3 for up to 8 seconds.

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

•	VIN 1--Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 7 seconds, then the A/F may go lean above 15.3 for up to 9 seconds.

•	Use the scan tool in order to verify whether DTC P0420 has passed or failed this key cycle.

The PCM determines that the catalysts oxygen storage capacity is below a threshold considered acceptable.

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

1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame/Failure Records data on the scan tool if applicable.

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 have caused its failure.

3. Clearing the DTCs allows the catalyst test to be completed up to 18 times and completed up to 6 times this ignition cycle. If the A/C is not turned OFF the diagnostic will not run. The engine must be warmed-up. The converter needs to be warmed-up by raising the engine speed above idle for the specified time prior to each attempted test. Check and see if 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 checks for conditions that can cause the three-way converter to appear degraded. Repair any conditions found before proceeding with this table.

9. If the three-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. These conditions may include misfire, leaking or plugged fuel injectors, high engine oil or coolant consumption, retarded spark timing or weak spark. Correct any possible causes of converter damage before replacing the converter.