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 PCM has the ability to monitor this process using the heated oxygen sensors. The Bank 1 HO2S 2 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 HO2S 3 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 HO2S 2 signal will be far more active than that produced by the Bank 1 HO2S 3 sensor. If the PCM detects a level of Bank 1 HO2S 3 activity that indicates the catalyst is no longer operating efficiently, DTC P0420 will be set.

The PCM determines if the catalyst is ready for testing based on the following criteria:

This ensures that the catalyst is sufficiently warmed up for testing purposes. Once these conditions have been met, the PCM can begin collecting the catalyst diagnostic test samples.

Check for the following conditions:

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Bank 1 HO2S 2/Bank 1 HO2S 3 Activity Test:

1. Ensure that the engine is fully warmed up.
2. Using a scan tool, monitor Bank 1 HO2S 2 and Bank 1 HO2S 3 voltage displays in Park while using the scan tool IAC RPM control function to maintain a mass air flow of 10 gm/s. You will need to run the engine in this manner for at least 3 minutes.
3. Compare the amount of activity (frequency and amplitude) on Bank 1 HO2S 2 to the activity on Bank 1 HO2S 3 during a 30 second period.
4. If the amount of activity on Bank 1 HO2S 3 is nearly as great as the activity on Bank 1 HO2S 2, a problem exists; use the DTC P0420 diagnostic table. If much less activity is noted on Bank 1 HO2S 3, the system is functioning properly.

The TWC Monitor Test Counter displayed on the scan tool may be used to monitor the progress of the TWC diagnostic. To complete the TWC diagnostic with a good catalyst, the counter must be allowed to increment to 49 samples and roll over to 0 at least once. A failed catalyst may require three or more 50 sample tests to report a failure.

Number(s) below refer to the step number(s) on the Diagnostic Table:

7. Difficulty completing the DTC P0420 test may be encountered in areas where test conditions cannot be maintained easily, especially in urban areas. To minimize the amount of driving required to complete the DTC P0420 test, use the following procedure:

1.	Allow the engine to warm completely.

2.	With the vehicle in Park, monitor mass air flow on the scan tool and hold part throttle to maintain a reading of over 12 gm/s for at least 2 minutes. This will achieve the warm catalyst requirement for running the test.

3.	Operate the vehicle in second or third gear to remain in the DTC P0420 test conditions as much as possible. Refer to Conditions For Setting the DTC.

4.	If you must stop the vehicle, maintain the warm catalyst criteria as follows:

0.4.1.

Place the vehicle in Park or Neutral.

0.4.2.

Hold part throttle to maintain a mass air flow reading of over 15 gm/s for the duration of the stop.

5.

The TWC Monitor Test Counter display on the scan tool may be used to monitor the progress of the TWC diagnostic. To complete the TWC diagnostic, the counter must be allowed to increment to 49 and roll over to 0 at least once. With a failed TWC, the counter may have to increment to 49 and roll over to 0 three times or more.