Catalytic converter
An exhaust component that converts the most harmful gases into less harmful ones.
A catalytic converter sits in the exhaust and treats the gases leaving the engine, cutting the output of carbon monoxide, unburned hydrocarbons and oxides of nitrogen. It is an emissions device rather than a performance one, and the test asks about it in the environmental section.
It needs the right fuel: leaded petrol destroys the catalyst, which is one of the historical reasons unleaded became universal. It also needs to be hot to work, so a series of very short journeys from cold produces disproportionate pollution.
The wider environmental questions on the paper follow the same logic — smooth driving, correct tyre pressures, not carrying unnecessary weight or an unused roof rack, and switching the engine off when stationary for any length of time all reduce consumption and emissions together.
6 practice questions on this
Each one shows the answer, and our note on why every other option is not it.