Three-Way Catalytic Converter Diagnostics

CATALYTIC CONVERTER DIAGNOSTICS

A. Heated Oxygen Sensor (ho2s) 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 in which HO (hydrocarbons) and CO (carbon monoxide) are oxidized and NOx (nitrogen oxides) are reduced. As the catalytic converter ages its capacity to store oxygen is impaired, leading to a decrease in its conversion capacity and the emission of unburned residues. To avoid this the Engine Control Module (ECM) monitors catalytic converter efficiency. This check is carried out as follows.

Two Heated Oxygen Sensors (HO2S) are used to check the catalytic converter 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 14 to 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 catalytic converter efficiency is high and the fuel/air mixture normal, the front sensor signal switches between rich and lean (large amplitude) while the rear sensor signal is even (small amplitude). When catalytic converter efficiency is poor and the fuel/air mixture normal, the rear sensor signal also switches between rich and lean (large amplitude) because exhaust gases are flowing through the 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, Diagnostic Trouble Code (DTC) EFI-443 Catalytic converter efficiency will be stored.