Code P0420, Catalyst System, Low O2 Storage Capacity Indicated: Notes

A three-way catalytic converter is used to maintain reasonably low 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, thus converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen and free oxygen.

PCM is capable of monitoring this process using HO2S No. 2. HO2S No. 2, which is located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively.

HO2S No. 2 signal will be far less active than that produced by HO2S No. 1 if catalyst is properly functioning. PCM will set Code P0420 if a problem exists which causes the PCM to detect excessive HO2S No. 2 activity outside of an acceptable range for an extended period of time, indicating that the three-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable.

Code P0420 will set when the following conditions are present.

• No MAF sensor, ECT sensor, HO2S No. 1 or HO2S No. 2 codes are set.
• Engine coolant temperature is greater than 185°F (85°C).
• Vehicle is in closed loop fuel control.
• Mass airflow is 12-40 grams per second.
• Above conditions are met for at least 2 consecutive minutes.
• Calculated engine load is steady.
• Vehicle speed is 30-68 MPH.
• PCM determines HO2S No. 2 activity to be excessive.
• All above conditions for up to 2 additional minutes after catalyst warm-up criteria has been met.

Code P0420 will not illuminate Malfunction Indicator Light (MIL).