Circuit Description

Three-Way Catalyst (TWC) system is used to control emissions. PCM uses signal from Heated Oxygen Sensors (HO2S) to read efficiency of TWC. PCM will set DTC P0420 if TWC oxygen storage capacity is less than a predetermined threshold.

Conditions required to set DTC are:

• No related DTCs set.
• Engine is operating in closed loop.
• Engine air load is less than 99.6 percent.
• ECT is greater than 140°F (60°C).
• MAF calculated airflow is 10-32 grams/second.
• Change in engine load is less than 3.91 percent.
• Catalyst temperature is greater than 750°F (399°C).
• Engine speed less than 3500 RPM.
• Vehicle speed is 15-75 MPH.
• PCM determines catalysts oxygen storage capacity less than predetermined threshold.