In order 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 the CO present in the exhaust gas, converting these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx, converting these chemicals into nitrogen. The converter also has the ability to store excess oxygen and to release the stored oxygen in order to promote these reactions. This oxygen storage capacity (OSC) is a measurement of the catalyst's 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 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 is considered to be bad and the malfunction indicator lamp (MIL) will be illuminated if subsequent tests also indicate a failure.

1. The conditions for engine warm-up have been met.

Use the scan tool catalyst data list in order to verify the following:

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DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0325, P0327, P0336, P0341, P0401, P0403, P0404, P0405, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1362, P1404, or P1441 are not set.

•	No exhaust gas recirculation (EGR), vehicle speed sensor (VSS), evaporative emission (EVAP) sensor, misfire, PRNDL, Trans, secondary air injection (AIR), or idle air control (IAC) valve DTCs set.

•	The engine has been running longer than 10 minutes.

•	The engine coolant temperature (ECT) is between 70-124°C (158-255°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 is less than 100°C (212°F).

•	The battery voltage is above 10.7 volts.

2. Warm the catalyst.

•	Fully open the hood.

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

•	Set the parking brake.

•	Press and hold the service brake.

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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 1,500-2,500 RPM. For any additional tests on the same key cycle, the engine must be between 1,500-2,500 RPM for 1 minute.

•	In order to activate the diagnostic, return the engine to idle, and place the automatic transmission in drive, or depress the clutch pedal for a manual transmission.

3. Test the catalyst.

•	Place the transaxle in drive for automatic transmissions, or in neutral with the clutch pedal depressed for manual transmissions.

•	Within 60 seconds, the air/fuel ratio will go lean, and should be above 15.3 for up to 7 seconds. The air/fuel ratio may then go rich, below 14.1 for up to 8 seconds.

•	Use the scan tool to check if DTC P0420 has passed or failed this key cycle.

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

Inspect for the following conditions:

If the condition is intermittent, refer to Intermittent 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 have caused its failure.

3. Clearing the DTCs allows the catalyst test to be completed up to 6 times during this ignition cycle. If the A/C is not turned OFF the diagnostic will 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. Check if the DTC passed or failed this ignition cycle. If the DTC does not pass or fail, look for a reason that would cause the test to abort.

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

9. If the 3-way converter needs to be replaced, make sure that another condition is not present which could damage the converter. These conditions may include: misfire, high engine oil or coolant consumption, retarded spark timing, weak spark, or a rich or lean fuel system. Correct any possible causes of converter damage before replacing the converter.