Catalytic Converter Diagnostic

The three-way catalytic converter 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 in which HC (hydrocarbons) and CO (carbon monoxide) are oxidized and NOx (nitrogen oxides) are reduced. As the three-way catalytic converter ages its ability to store oxygen drops. This reduces the conversion capacity of the three-way catalytic converter. To avoid dangerous emissions the ECM checks three-way catalytic converter efficiency. This check is carried out as follows.

Two heated oxygen sensors are used to check the three-way catalytic converter one upstream and one downstream of the catalytic converter. The main function of the heated oxygen sensors is to measure the oxygen content in the exhaust gases so that the ECM can maintain the fuel/air mixture at around lambda equals 1. This mixture provides optimum three-way catalytic converter efficiency.

To determine catalytic converter efficiency the amplitude of the HO2S signals is compared (amplitude is a measure of signal oscillation). When three-way catalytic converter efficiency is good and the fuel/air mixture is normal, the front HO2S signal switches between rich and lean while the rear HO2S signal is even. When three-way catalytic converter efficiency is poor and the fuel/air mixture is normal, the rear HO2S signal switches between rich and lean because the exhaust gases are passing straight through the three-way catalytic converter without being converted.

If rear sensor amplitude becomes too great in relation to the front sensor signal for a number of diagnostic checks, the following DTCs will be stored.

1. ECM-4801
   
   5-cylinder engines and 6-cylinder engines, bank 1 (cylinder Nos. 1-3)
2. ECM-4901
   
   6-cylinder engines bank 2 (cylinder Nos. 4-6)