Three-Way Catalytic Converter (TWC) Diagnostics

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.
To be able to check the catalytic converter, two heated oxygen sensors are used; one heated oxygen sensor in front of the catalytic converter in the engine compartment (front heated oxygen sensor) and a heated oxygen sensor after the catalytic converter in the engine compartment (rear heated oxygen sensor). After the rear heated oxygen sensor there is another catalytic converter, located at the front end of the propeller shaft tunnel under the vehicle's floor. However, this catalytic converter is not monitored.
The unique thing with this version (PZEV) is that there is a third heated oxygen sensor (middle heated oxygen sensor), located at the front on the catalytic converter in the engine compartment, that is, between the front and rear heated oxygen sensor. However, the middle heated oxygen sensor is not used to perform catalytic converter diagnostics, it has another purpose.
The main function of the heated oxygen sensors is to measure the oxygen content in the exhausts 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.
In practise this means that catalytic converter diagnosis starts within 20 minutes after engine start (time varies depending on market).
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 centre and rear heated oxygen sensors register a large number of switches exceeding a parameter, a diagnostic trouble code for catalytic converter efficiency will be stored.
If the parameter is not exceeded the three-way catalytic converter (TWC) is deemed to be operating well.