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 the NOx, converting it 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 powertrain control module (PCM) monitors this process using a heated oxygen sensor (HO2S) that is 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 bad 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:

•	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)-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.

•	Ensure that the hood is fully open.

•	Set the transmission in Park for automatic transmissions, or 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 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.

•	To activate the diagnostic, return to idle and place the vehicle in Drive for automatic transmissions, or depress the clutch pedal for manual transmissions.

3. Test the catalyst.

•	Ensure that the transaxle is in Drive for automatic transmissions, or in Neutral with the clutch pedal depressed for manual transmissions.

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

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

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

Inspect for the following conditions:

Important:  Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

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

1. The Powertrain On Board Diagnostic (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 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 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 three-way converter to appear degraded. Repair any problems 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, high engine oil or coolant consumption, retarded spark timing or weak spark. Correct any possible causes of converter damage before replacing the converter.