Three-Way Catalytic Converter (TWC) Diagnostics

Three-way catalytic converter (TWC) diagnostics

The catalytic converter stores oxygen from the exhaust gases and uses it to make toxic gases more environmentally friendly. The catalytic converter is a three-way catalytic converter (TWC) in which HC (hydrocarbons) and CO (carbon monoxide) are oxidized and NOx (nitrous oxide) is reduced. As the catalytic converter ages, its ability to store oxygen is reduced. The conversion capacity of the catalytic converter is reduced and unburned residue which is harmful to environment is released. To reduce the environmentally damaging emissions, the engine control module (ECM) checks the efficiency of the catalytic converter. In brief, this check is carried out as follows.
Two heated oxygen sensors (HO2S) are used to check the three-way catalytic converter (TWC), one upstream of the converter and one in the center of the converter (the rear heated oxygen sensor (HO2S)). The main function of the heated oxygen sensors (HO2S) is to measure the oxygen content in the exhaust gases so that the engine control module (ECM) can maintain the fuel / air mixture at around lambda=1. This mixture provides optimum three-way catalytic converter (TWC) efficiency. To determine catalytic converter efficiency the amplitude of the heated oxygen sensor (HO2S) signals is compared (amplitude is a measure of signal oscillation).
When efficiency of the catalytic converter is good and the fuel / air mixture is normal, the front probe signal switches between rich and lean while the rear probe signal is steady. If the three-way catalytic converter (TWC) efficiency is poor but the fuel / air mixture is normal, the rear probe signal will also switch between rich and lean. This is because the exhaust gases flow straight through the three-way catalytic converter (TWC) without being acted on. A diagnostic trouble code (DTC) for catalytic converter efficiency will be stored if rear sensor amplitude becomes too great in relation to the front sensor signal for a number of diagnostic checks.