Circuit Description

Three-Way Catalyst (TWC) is used to control carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) exhaust emissions. TWC promotes a chemical reaction which oxidizes HC and CO, converting them to water vapor and carbon dioxide. NOx emissions are converted to nitrogen. PCM monitors this process using a Heated Oxygen Sensor No. 2 (HO2S2) located after TWC. PCM compares HO2S2 signal with Heated Oxygen Sensor No. 1 (HO2S1) located before TWC. If TWC is operating correctly, HO2S2 signal will be much less active than HO2S1. When both oxygen sensor signals change at similar rates, deteriorated catalyst performance is indicated.

DTC will set when HO2S2 and HO2S1 signals change at similar rates with engine operating in closed loop, engine coolant temperature greater than 158°F (70°C), and engine speed is 2500-3000 for at least 3 minutes while driving at certain vehicle speeds.

When DTC sets, MIL will illuminate after 2 consecutive ignition cycles in which diagnostic runs with active fault. PCM will record operating conditions at time of diagnostic failure and store information in freeze frame buffer.