The FAP is the 'Filtre à particules' which is Citroen term for 'Diesel Particulate Filter'
In normal running the FAP is regenerated every so often during driving. When the computer has sensed the correct driving conditions and the state of the filter the injector timing is altered and Eolys/Infineum/Powerflex/Rhodia DPX42 fluid is injected into the fuel tank to alter the burn characteristics of the fuel to raise the exhaust temperature enough to regenerate the filter.
The problem you may have encountered is that the computer never sees the correct driving conditions (lots of short journeys so the system doesn't get up to temperature) or the fluid is exhausted (refill kits available here).
You may also have problems if you only put a small quantity of fuel in at a time, rather that filling up as a high fuel tank level may be required to trigger the automatic regeneration.
In any case, the quick fix to regenerating the filter (and restoring filter performance) is to force a regeneration with a supplemental dose of this concentrated Diesel particulate filter cleaner from Amazon.
This doesn't refill the fuel tank additive container with Eolys type fluid (available here), so if the tank is empty, it should be refilled at some point. If the Anti-pollution Fault was due to short journeys, the problem may disappear if the driving routine changes.
For ongoing assistance to the DPF regeneration and cleaning, particularly useful if you don't drive the car often enough in a way that meets the DPF regen threshold, an addition of Regen Plus+directly into the fuel tank at every fill-up of fuel is useful. The regular dose will bypass the computer control and force the filter to regenerate at a lower temperature as you drive can save a huge repair bill for the DPF when it fails the MOT test.
