2002 Citroen C5 Fault code

My c5 auto has come up with a service light and an information mesage stating "Anti pollution level low" I can find nothing in the hand book about any fuel additive or anti pollution level checks that I can do. The local Citroen garage where I had the vehicle serviced 2 weeks ago are not prepared to even have a look unless I pay £65.00. Any advice greatfully received.

Thanks,

D