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-2 (HO2S-2), located after TWC. PCM compares HO2S-2 signal with Heated Oxygen Sensor-1 (HO2S-1), located before TWC. If TWC is operating correctly, HO2S-2 signal will be much less active than HO2S-1. When both oxygen sensor signals change at similar rates, deteriorated catalyst performance is indicated.

DTC will set when HO2S-2 and HO2S-1 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.