Refer to Engine Controls Schematics .

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 catalysts ability to control emissions. The 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 HO2S1 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 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 VSS, EVAP, Misfire, HO2S, EGR, ECT, MAF, ETC, IAT, MAP, CMP, CKP sensor, Fuel Trim, or Fuel Injector DTCs set.

•	Engine has been running longer than 10 minutes.

•	Engine coolant temperature (ECT) is more than 76°C (169°F) and less than 124°C (255°F).

•	Baro is more than 75 kPa.

•	Vehicle is in Closed Loop.

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

•	Battery voltage more than 10.7 volts.

2. Warm up the catalyst.

•	Fully open hood.

•	Transmission is in park (automatic) or neutral (manual).

•	Set the parking brake.

•	Press and hold service brake.

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Each time the engine is started the diagnostic can run up to 18 times. After the ten minute run time, and before the diagnostic runs the first time, the engine must run an additional 5 minutes between 1500 and 2000 RPM. Any additional tests on the same key cycle, the RPM must be between 1500 and 2000 RPM for 1 minute.

•	To activate the diagnostic; Return to idle and put the vehicle in drive (automatic) or depress the clutch (manual).

3. Test the catalyst.

•	Place the transmission in drive (automatic) or neutral and depress the clutch (manual).

•	Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 5 seconds, then it may go lean above 15.3 for up to 6 seconds.

•	Using the scan tool check and see if DTC P0420 has passed or failed this key cycle

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

Inspect for the following:

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.

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 tests and store the freeze frame and failure records data on the scan tool if applicable.

2. If any other 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 attempted 18 times and completed up to 6 times this key cycle. If the A/C is ON the diagnostic may not run. The engine must be warmed-up. The converter needs to be warmed-up by raising the engine speed more than idle for the specified time prior to each attempted test. Check and see if DTC passed or failed this key 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, weak spark or a plugged/leaking injectors. Correct any possible causes of converter damage before replacing the converter.