Catalytic Converter: Description and Operation

Three-way catalytic converter (TWC) diagnostics

The three-way catalytic converter (TWC) stores oxygen from the exhaust gases and uses it to make toxic gases more environmentally friendly. The catalytic converter is a 3 way catalytic converter in which HC (hydrocarbons) and CO (carbon monoxide) are oxidized and NOx(nitrous oxide) is reduced. As the three-way catalytic converter (TWC) ages, its ability to store oxygen is reduced. The conversion capacity of the three-way catalytic converter (TWC) 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 three-way catalytic converter (TWC). 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 is upstream of the converter and one is 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 allows for optimum catalytic conversion. To determine catalytic converter efficiency the amplitude of the heated oxygen sensor (HO2S) signals is compared (amplitude is a measure of signal oscillation).
When the efficiency of the catalytic converter is good and the fuel/air mixture is normal, the signal of the front heated oxygen sensor (HO2S) switches between rich and lean while the rear heated oxygen sensor (HO2S) is steady. If the efficiency of the three-way catalytic converter (TWC) is poor but the fuel/air mixture is normal, the signal from the rear heated oxygen sensor (HO2S) 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. If the amplitude of the rear heated oxygen sensor (HO2S) becomes too great in relation to the amplitude of the front sensor for a number of checks, a diagnostic trouble code (DTC) will be stored for catalytic converter efficiency.