To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The TWC 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 TWC. 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 be illuminated if subsequent tests also indicate a failure.

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

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DTCs P0030, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0404, P0405, P0440, P0442, P0443, P0502, P0503, P0506, P0507, P1133, P1134, P1351, P1352, P1361, P1362, or P1441 are not set.

•	The engine has been running longer than 10 minutes.

•	The engine coolant temperature (ECT) is between 76-124°C (169-255°F).

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

•	The vehicle is in Closed Loop.

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

•	The battery voltage above 10.7 volts.

2. Warm up the catalyst.

•	Fully open the hood.

•	The transmission is in Park, automatic, or Neutral, manual.

•	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 vehicle in drive, or depress the clutch pedal for a manual transmission.

3. Test the catalyst.

•	The transaxle is in drive, for automatic transaxles, or neutral for manual transaxles with the clutch pedal depressed.

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

•	Verify if DTC P0420 has passed or failed this key cycle with a scan tool.

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

Inspect for the following conditions:

If the condition is determined to be 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 TWC to appear degraded or may have caused its failure.

3. Clearing the DTCs allows the catalyst test to be 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 warmed-up. The TWC needs to be warmed-up by raising the engine speed above idle for the specified time prior to each attempted test. Inspect and see 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 tests for conditions that can cause the TWC to appear degraded. Repair any conditions found before proceeding with this table.

9. If the TWC needs to be replaced, make sure that another condition is not present which could damage the TWC. 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 TWC damage before replacing the TWC.