Three-Way Catalytic Converter (TWC) Diagnostics

Three-way catalytic converter diagnostics

Three-way catalytic converter 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 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.