Catalytic Converter: Description and Operation

Three-way catalytic converter (TWC) diagnostics
The three-way catalytic converter (TWC) stores oxygen from 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 are used to check the three-way catalytic converter (TWC). One is upstream of the converter and one is in the center of the converter (the rear heated oxygen sensor).
The main function of the heated oxygen sensors 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 allows for optimum catalytic conversion.
To evaluate the efficiency of the three-way catalytic converter (TWC) one uses a deviation added to the lambda signal.
This means that 8 to 15 minutes after the engine is started (depending on the market) catalytic converter diagnostics start.
The process is as follows:
A deviation is added to the lambda signal and the engine control module (ECM) is allowed to correct this.
The deviation switches between positive and negative values so that the fuel/air mixture switches between rich and lean.
A counter registers the number of switches occurring while the diagnostic is taking place.
If the rear heated oxygen sensor registers a large number of switches exceeding a parameter, a diagnostic trouble code (DTC) for three-way catalytic converter (TWC) efficiency will be stored.
If the parameter is not exceeded the three-way catalytic converter (TWC) is deemed to be operating well.

Note! In a poorly operating three-way catalytic converter (TWC) the switches caused by the deviation will quickly exceed the parameter. This is because the exhaust gases in principle pass straight through the three-way catalytic converter (TWC) without being cleaned. If the three-way catalytic converter (TWC) is operating well the switches will be slower as it takes a longer time for the variations to make themselves felt.

There are four catalytic converters, two for bank 1 and two for 2.
Bank 1; cylinder 1, 2, and 3.
Bank 2; cylinder 4, 5, and 6.