A three-way catalytic converter is used In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). 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 then to 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 PCM monitors this process using a heated oxygen sensor (HO2S), which is located in the exhaust stream behind the three-way converter. When the catalyst is functioning properly, HO2S 2 is slow to respond to a large change in HO2S 1 signal. When HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst is considered to be faulty and the MIL will illuminate 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:

•	No , EGR, ECT, MAF, TP, IAT, MAP, CMP or CKP sensor, VSS, EVAP, misfire, HO2S, IAC valve, fuel trim, or fuel injector DTCs are set.

•	The engine has been running for more than 10 minutes

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

•	The BARO is more than 75 kPa

•	The vehicle is in closed loop.

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

•	The battery voltage is more than 10.7 volts.

2. Warm up the catalyst and check the following:

•	The hood is fully open.

•	The transmission is:

-	In Park if the vehicle has an automatic transmission

-	In Neutral if the vehicle has a manual transmission

•	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 1500 and 2500 RPM. For any additional tests on the same key cycle, the engine must be between 1500 and 2500 RPM for 1 minute.

•	In order to activate the diagnostic return the engine to idle and:

-	Place the transmission in Drive if the vehicle has an automatic transmission.

-	Press the clutch pedal and place the transmission in Neutral if the vehicle has a manual transmission.

3. Test the catalyst.

-	Place the transmission in Drive if the vehicle has an automatic transmission.

-	Press the clutch pedal and place the transmission in Neutral if the vehicle has a manual transmission.

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

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

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

Check for the following:

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

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and to store the Freeze Frame and 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 6 times during 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. Verify whether the DTC passed or failed this ignition cycle. If the DTC does not pass or fail, inspect 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 problems that you find before proceeding with this table.

9. If the three-way converter needs to be replaced, be 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, or weak spark. Correct any possible causes of converter damage before you replace the converter.