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 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 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 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 set.

•	Engine has been running longer than 10 minutes.

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

•	BARO is above 75 kPa.

•	Vehicle is in Closed Loop.

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

•	Battery voltage is above 10.7 volts.

2. Warm up the catalyst.

•	Fully open hood.

•	Transmission is in park, for automatics, or neutral, for manuals.

•	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. 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, depress the clutch pedal for manual.

3. Test the catalyst.

•	Transaxle is in drive, for automatics, or neutral for manuals with clutch pedal depressed.

•	Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 7 seconds, then may go lean above 15.3 for up to 10 seconds.

•	Using the scan tool check and see 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.

Inspect for the following conditions:

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

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 condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

Numbers below refer to the step numbers on the Diagnostic Table:

1. The powertrain OBD system check prompts the technician to complete some basic tests and 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. Observe the 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 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.