2002 Citroen C5 anti-polution

You have a clogged catalytic convertor. It may need replacing. Start by having the computer reset and do the following drive cycle !

Diesel Particulate Filters (DPFs) Or Anti polution Filter

Particulates, or soot particles are the most visible of the pollutants emitted from the exhaust of a diesel engine, they consist of large carbon particles with other attached and absorbed chemicals. Particulates in addition to being visible are toxic and carcinogenic, and the smaller particle of less than 10 microns (known as PM10s) can penetrate deep into the lungs causing respiratory problems. With the increased use of diesel engines, the increase in air borne particulates is becoming an increasing environmental issue.

These solid emissions can be effectively reduced by filtration of the exhaust. The most effective and practical method is by using a ceramic filter sustrate, usually silicone carbide with microporus walls. This traps the solid particles and allows the gasses to escape through the wall and is vented into the exhaust outlet.
The collected soot builds up within the filter, which will clog and eventually block resulting in a build up of engine back pressure unless it is removed, which is achieved by a process known as regeneration.
Regeneration is achieved by burning the collected soot within the filter, either actively or passively. In an active system a separate, usually electrical, heat source is applied to raise the temperature to a point where the carbon particles are burnt off to produce CO2. In the passive system the heat of the engine exhaust gas is used in conjunction with a special catalytic coating on the filter to achieve the same result

when the anti pollution light came on! It was simply telling you that the Diesel Particulate Filter (DPF) was almost full of soot and the car needed to be driven for 30 miles or so at 40 - 50 mph or above to burn off the accumulated soot and clear the filter. This is not a fault with the car - simply a feature of cars with a DPF. (I often get asked why you can't simply change the filter rather than wasting fuel going for a long drive to clear it. The answer is, you can. The cost is between  1000/  2000, which makes that drive look suddenly economical).