Could be the sender uint in the tank, could be the cars software, without a trip to a Peugeot dealer you aint going to know which it is.
Hope it aint the tank sender as they are expensive to change as the tank has to be dropped.
The 406 was never designed to have HDI power & thats why the hole under the rear seat isnt big enough to get a HDI tank sender / pump unit through.
Lee
June 2007