Fuel addative fault

[Peter86]

Good afternoon guys, im new to the forum, and new to owning a 406. I have a 2001 2.2 executive estate.
I have been having a "fuel addative" fault coming on since i bought it, and there is a light on the dash board. The previous owner said he has changed all the sensors involved and gave up with it in the end. This has no effect on the running of the car, it runs spot on, but i am worried if i ignore it, i will be forking out for a CAT in the near future?
Any suggestions what this is?
Thanks in advance