Three-Way Catalytic Converter (TWC) Diagnostics

Three-way catalytic converter (TWC) diagnostics




The three-way catalytic converter (TWC) stores oxygen found in 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 upstream and one downstream of the catalytic converter. 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 three-way catalytic converter (TWC) efficiency is good and the fuel-/ air mixture is normal, the front heated oxygen sensor (HO2S) signal switches between rich and lean while the rear heated oxygen sensor (HO2S) signal is even. When three-way catalytic converter (TWC) efficiency is poor and the fuel-/ air mixture is normal, the rear heated oxygen sensor (HO2S) signal switches between rich and lean because the exhaust gases are passing straight through the three-way catalytic converter (TWC) without being converted.
If the amplitude of the rear sensor 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.