Catalytic Converter Diagnostics

Catalytic Converter Diagnostic
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 three way catalytic converter (TWC) in which HC (hydrocarbons) and CO (carbon monoxide) are oxidized and NOx (nitrogen oxides) are reduced. As the three-way catalytic converter (TWC) ages its ability to store oxygen drops. This reduces the conversion capacity of the three-way catalytic converter (TWC). To avoid dangerous emissions the engine control module (ECM) checks three-way catalytic converter (TWC) efficiency. 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 rear sensor amplitude becomes too great in relation to the front sensor signal for a number of diagnostic checks, diagnostic trouble code (DTC) ECM-4801 Catalytic converter efficiency will be stored.