The older PSA engined cars usually had a sealed canister hung under the car which is disposable and dosed the catalyst at preset intervals. Once empty it needs replacing (a light comes on the dash). They are however a regular service item and are changed as a matter of course at major services so most folk who get their car dealer serviced never see the light or have anything to do with the system. You don't have to fill them up as such.

The latest Euro 6 engines need much more urea injected than previous versions to regenerate the catalyst(s) more often to maintain lower emissions so the move has been towards refillable tanks in the last couple of years.

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