Catalytic Converter Diagnostics

THREE-WAY CATALYTIC CONVERTER DIAGNOSTICS

The Three-way catalytic converter (TWC) stores the oxygen present in the exhaust gases and uses it to render the toxic constituents less environmentally hazardous. The unit is of the three-way type in which the hydrocarbons (HC) and carbon monoxide (CO) are oxidized and the nitrogen oxides (NOx) are reduced. Aging of the TWC can reduce its oxygen storage capacity, impairing its ability to perform its function and allowing the escape of environmentally hazardous unburned residues. As described briefly below, the ECM monitors the efficiency of the TWC to prevent the escape�of hazardous residues.

Two HO2S sensors are used to monitor the TWC, one upstream and the other downstream of the converter. Their main function is to measure the oxygen content of the exhaust gases, to enable the ECM to maintain the air/fuel ratio at a value corresponding to lambda = 1, at which conditions for converting the noxious constituents in the exhaust gases are optimum. The amplitudes of the HO2S signals (the amplitude being a measure of the signal variation) are compared in order to monitor The efficiency of the unit.

When the TWC is functioning satisfactorily and the air/fuel ratio is normal, the front HO2S signal varies between rich and lean (at a high amplitude), while the rear HO2S signal should remain steady (at a low amplitude). When the TWC efficiency is poor and the air/fuel ratio is normal, the rear HO2S signal will also vary from rich to lean (at a high amplitude) since the gases will flow through the TWC without being treated.

DTC 4-4-3 TWC efficiency' will be recorded if the amplitude of the rear HO2S signal remains high relative to the front HO2S signal during a number of successive checks.