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 has the ability to monitor catalyst efficiency by using HO2S-1 and HO2S-2. HO2S-1 produces an output signal indicating oxygen content in exhaust gases entering Three-Way Catalyst (TWC). HO2S-2 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, HO2S-1 will be far more active than HO2S-2.

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/FAILURE RECORDS data buffer.