Circuit Description

PCM has the ability to monitor catalyst efficiency by using HO2S1 and HO2S2. HO2S1 produces an output signal indicating oxygen content in exhaust gases entering TWC. HO2S2 produces an output signal indicating oxygen storage capacity of TWC. A comparison reading from sensors determines catalyst's ability to convert exhaust gases efficiently. If TWC is operating efficiently, HO2S1 will be far more active than HO2S2. DTC P0420 will set when:

• HO2S2 activity is more than a specified value.
• HO2S2 voltage is within a certain percentage of HO2S1.